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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1 files changed, 9 insertions, 0 deletions

diff --git a/components/script/microtask.rs b/components/script/microtask.rs
index 7fadf111ddb..8f10dbcf489 100644
--- a/components/script/microtask.rs
+++ b/components/script/microtask.rs
@@ -11,6 +11,7 @@ use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::PromiseBinding::PromiseJobCallback;
 use dom::bindings::js::Root;
 use dom::globalscope::GlobalScope;
+use dom::htmlmediaelement::MediaElementMicrotask;
 use dom::mutationobserver::MutationObserver;
 use msg::constellation_msg::PipelineId;
 use std::cell::Cell;
@@ -29,9 +30,14 @@ pub struct MicrotaskQueue {
 #[derive(JSTraceable, HeapSizeOf)]
 pub enum Microtask {
     Promise(EnqueuedPromiseCallback),
+    MediaElement(MediaElementMicrotask),
     NotifyMutationObservers,
 }
 
+pub trait MicrotaskRunnable {
+    fn handler(&self) {}
+}
+
 /// A promise callback scheduled to run during the next microtask checkpoint (#4283).
 #[derive(JSTraceable, HeapSizeOf)]
 pub struct EnqueuedPromiseCallback {
@@ -72,6 +78,9 @@ impl MicrotaskQueue {
                         if let Some(target) = target_provider(job.pipeline) {
                             let _ = job.callback.Call_(&*target, ExceptionHandling::Report);
                         }
+                    },
+                    Microtask::MediaElement(ref task) => {
+                        task.handler();
                     }
                     Microtask::NotifyMutationObservers => {
                         MutationObserver::notify_mutation_observers();