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/document.rs

@@ -121,6 +121,7 @@ use crate::dom::element::{
 use crate::dom::event::{Event, EventBubbles, EventCancelable, EventDefault, EventStatus};
 use crate::dom::eventtarget::EventTarget;
 use crate::dom::focusevent::FocusEvent;
+use crate::dom::fontfaceset::FontFaceSet;
 use crate::dom::globalscope::GlobalScope;
 use crate::dom::gpucanvascontext::{GPUCanvasContext, WebGPUContextId};
 use crate::dom::hashchangeevent::HashChangeEvent;
@@ -466,6 +467,9 @@ pub struct Document {
     /// - <https://bugzilla.mozilla.org/show_bug.cgi?id=1596992>
     /// - <https://github.com/w3c/csswg-drafts/issues/4518>
     resize_observers: DomRefCell<Vec<Dom<ResizeObserver>>>,
+    /// The set of all fonts loaded by this document.
+    /// <https://drafts.csswg.org/css-font-loading/#font-face-source>
+    fonts: MutNullableDom<FontFaceSet>,
 }
 
 #[derive(JSTraceable, MallocSizeOf)]
@@ -3291,6 +3295,7 @@ impl Document {
             pending_compositor_events: Default::default(),
             mouse_move_event_index: Default::default(),
             resize_observers: Default::default(),
+            fonts: Default::default(),
         }
     }
 
@@ -5364,6 +5369,12 @@ impl DocumentMethods for Document {
             None
         }
     }
+
+    // https://drafts.csswg.org/css-font-loading/#font-face-source
+    fn Fonts(&self) -> DomRoot<FontFaceSet> {
+        self.fonts
+            .or_init(|| FontFaceSet::new(&*self.global(), None))
+    }
 }
 
 fn update_with_current_time_ms(marker: &Cell<u64>) {