script: Implement the FontFaceSet DOM API (#32576)

Add a skeleton implementation of FontFaceSet interface with support
for resolving the `document.fonts.ready` Promise when the loading of
web fonts is completed.

This change exposes new failures in the web platform tests.

These were ERROR before the change because `document.fonts.ready` caused
a `ReferenceError` causing the tests to be aborted and they now FAIL:
- /css/CSS2/linebox/vertical-align-top-bottom-001.html
- /css/css-flexbox/flex-one-sets-flex-basis-to-zero-px.html
- /css/css-fonts/generic-family-keywords-001.html
- /css/css-fonts/math-script-level-and-math-style/math-script-level-004.tentative.html
- /css/css-fonts/math-script-level-and-math-style/math-script-level-002.tentative.html
- /css/css-text/text-autospace/text-autospace-ligature-001.html
- /css/css-values/calc-size/calc-size-width.tentative.html

These were TIMEOUT before the change because `document.fonts.ready` was
a ReferenceError and the tests were asynchronous (reftest-wait). These now
FAIL because the assertions are now executed after fonts are loaded:
- /css/css-fonts/matching/fixed-stretch-style-over-weight.html
- /css/css-fonts/matching/range-descriptor-reversed.html
- /css/css-fonts/matching/stretch-distance-over-weight-distance.html
- /css/css-fonts/matching/style-ranges-over-weight-direction.html
- /css/css-fonts/variations/variable-box-font.html
- /css/css-fonts/variations/variable-gpos-m2b.html
- /css/css-fonts/variations/variable-gsub.html
- /css/css-fonts/variations/variable-opsz-size-adjust.html
- /css/css-position/sticky/position-sticky-change-top.html
- /css/css-position/sticky/position-sticky-fixed-ancestor.html
- /css/css-position/sticky/position-sticky-flexbox.html
- /css/css-position/sticky/position-sticky-grid.html
- /css/css-position/sticky/position-sticky-inline.html
- /css/css-position/sticky/position-sticky-rendering.html
- /css/css-position/sticky/position-sticky-stacking-context.html
- /css/css-position/sticky/position-sticky-table-td-left.html
- /css/css-position/sticky/position-sticky-table-td-right.html
- /css/css-position/sticky/position-sticky-table-tfoot-bottom.html
- /css/css-position/sticky/position-sticky-table-th-right.html
- /css/css-position/sticky/position-sticky-table-thead-top.html
- /css/css-position/sticky/position-sticky-table-tr-bottom.html
- /css/css-position/sticky/position-sticky-table-tr-top.html
- /css/css-position/sticky/position-sticky-writing-modes.html
- /css/css-pseudo/marker-intrinsic-contribution-001.html
- /css/css-text/hyphens/hyphens-character.html

These tests now PASS due to this patch:
* FAIL -> PASS
  - /html/canvas/element/text/2d.text.draw.fill.maxWidth.fontface.html
  - /html/canvas/element/text/2d.text.measure.width.empty.html
* TIMEOUT -> PASS
  - /css/css-fonts/variations/font-descriptor-range-reversed.html
  - /css/css-fonts/variations/variable-opsz.html
  - /css/css-position/sticky/position-sticky-table-th-left.html
* ERROR -> PASS
  - /css/css-fonts/generic-family-keywords-002.html
  - /css/css-fonts/generic-family-keywords-003.html
* These two tests only PASS in Layout 2020:
  - /css/CSS2/positioning/inline-static-position-001.html
  - /css/cssom-view/getBoundingClientRect-empty-inline.html

These two tests have subtests that PASS intermittenttly:
- /fetch/metadata/generated/css-font-face.sub.tentative.html
- /css/css-fonts/generic-family-keywords-001.html

These tests are new TIMEOUTS that used to FAIL because
`documents.fonts.ready` was undefined:
- /resource-timing/TAO-match.html
- /resource-timing/content-type.html
- /resource-timing/nextHopProtocol-is-tao-protected.https.html

The failure in `/resize-observer/change-layout-in-error.html` could be
due to an issue in the ResizeObserver implementation that is now exposed
with this change, but this needs more investigation.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/window.rs

@@ -1952,6 +1952,10 @@ impl Window {
     /// may happen in the only case a query reflow may bail out, that is, if the
     /// viewport size is not present). See #11223 for an example of that.
     pub fn reflow(&self, reflow_goal: ReflowGoal, reason: ReflowReason) -> bool {
+        // Fetch the pending web fonts before layout, in case a font loads during
+        // the layout.
+        let pending_web_fonts = self.layout.borrow().waiting_for_web_fonts_to_load();
+
         self.Document().ensure_safe_to_run_script_or_layout();
         let for_display = reflow_goal == ReflowGoal::Full;
 
@@ -1980,6 +1984,24 @@ impl Window {
             );
         }
 
+        let document = self.Document();
+        let font_face_set = document.Fonts();
+        let is_ready_state_complete = document.ReadyState() == DocumentReadyState::Complete;
+
+        // From https://drafts.csswg.org/css-font-loading/#font-face-set-ready:
+        // > A FontFaceSet is pending on the environment if any of the following are true:
+        // >  - the document is still loading
+        // >  - the document has pending stylesheet requests
+        // >  - the document has pending layout operations which might cause the user agent to request
+        // >    a font, or which depend on recently-loaded fonts
+        //
+        // Thus, we are queueing promise resolution here. This reflow should have been triggered by
+        // a "rendering opportunity" in `ScriptThread::handle_web_font_loaded, which should also
+        // make sure a microtask checkpoint happens, triggering the promise callback.
+        if !pending_web_fonts && is_ready_state_complete {
+            font_face_set.fulfill_ready_promise_if_needed();
+        }
+
         // If writing a screenshot, check if the script has reached a state
         // where it's safe to write the image. This means that:
         // 1) The reflow is for display (otherwise it could be a query)
@@ -1989,8 +2011,6 @@ impl Window {
         // that this pipeline is ready to write the image (from the script thread
         // perspective at least).
         if self.prepare_for_screenshot && for_display {
-            let document = self.Document();
-
             // Checks if the html element has reftest-wait attribute present.
             // See http://testthewebforward.org/docs/reftests.html
             let html_element = document.GetDocumentElement();
@@ -1998,9 +2018,7 @@ impl Window {
                 elem.has_class(&atom!("reftest-wait"), CaseSensitivity::CaseSensitive)
             });
 
-            let pending_web_fonts = self.layout.borrow().waiting_for_web_fonts_to_load();
             let has_sent_idle_message = self.has_sent_idle_message.get();
-            let is_ready_state_complete = document.ReadyState() == DocumentReadyState::Complete;
             let pending_images = !self.pending_layout_images.borrow().is_empty();
 
             if !has_sent_idle_message &&