script: add skeleton implementation of FontFace API (#35262)

This patch implements the `FontFace` interface, but with some caveats

1. The interface is only exposed on `Window`. Support for Workers will
   be handled in the future.
2. The concept of `css-connected` `FontFace` is not implemented, so
   `@font-face` rules in stylesheets will not be represented in the DOM.
3. The constructor only supports using `url()` strings as source
   and `ArrayBuffer` and `ArrayBufferView` are not supported yet.

A skeleton implementation of the `load` method of `FontFaceSet` is also
implemented in this patch. The intention is to support some web pages
that don't load without this method.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/script_thread.rs

@@ -1614,6 +1614,9 @@ impl ScriptThread {
                 ScriptThreadEventCategory::FileRead => {
                     time_profile!(ProfilerCategory::ScriptFileRead, None, profiler_chan, f)
                 },
+                ScriptThreadEventCategory::FontLoading => {
+                    time_profile!(ProfilerCategory::ScriptFontLoading, None, profiler_chan, f)
+                },
                 ScriptThreadEventCategory::FormPlannedNavigation => time_profile!(
                     ProfilerCategory::ScriptPlannedNavigation,
                     None,