Auto merge of #16861 - gterzian:use_microtask_to_await_stable_state, r=jdm

Use microtasks to await a stable state.

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@jdm @KiChjang First pass at using microtasks to await a stable state. I ran into all sorts of problems to get it to compile, I think it's mainly related to the fact that the microtasks are stored in a `Vec`, which meant the `Runnalbe.handler(self: Box<Self>)` couldn't be called while iterating over the Vec... It compiles now although I haven't run any tests. I'm assuming I'm missing something fundamental and was hoping my changes would highlight the problems I run into, and you had a better idea of how to implement this... Perhaps we shouldn't pass a `Runnable` to `await_stable_state` at all?

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15375 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-in-sync-event.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-in-sync-event.html]
-  type: testharness
-  [await a stable state and sync event handlers]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-load.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-load.html]
-  type: testharness
-  [invoking resource selection with load()]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-pause-networkState.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-pause-networkState.html]
-  type: testharness
-  [NOT invoking resource selection with pause() when networkState is not NETWORK_EMPTY]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-pause.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-pause.html]
-  type: testharness
-  [invoking resource selection with pause()]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-play.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-play.html]
-  type: testharness
-  [invoking resource selection with play()]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-remove-from-document-networkState.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-remove-from-document-networkState.html]
-  type: testharness
-  [NOT invoking resource selection with implicit pause() when networkState is not NETWORK_EMPTY]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-set-src-not-in-document.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-set-src-not-in-document.html]
-  type: testharness
-  [invoking load by setting src on video not in a document]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-invoke-set-src.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-invoke-set-src.html]
-  type: testharness
-  [invoking load by setting src]
-    expected: FAIL
-

tests/wpt/metadata/html/semantics/embedded-content/media-elements/loading-the-media-resource/resource-selection-remove-src.html.ini

@@ -1,5 +0,0 @@
-[resource-selection-remove-src.html]
-  type: testharness
-  [invoking resource selection by setting src; await stable state]
-    expected: FAIL
-