Code health cleanup: replacing animations.

Animation::Update performed a synchronous processing of the finish
microtask to ensure that finished events where dispatched ahead of
replace events. This step does not align with the spec.  Instead we
should be queuing the replace event.  Microtasks will be processed in
the correct order.

Spec link: https://www.w3.org/TR/web-animations-1/#timelines


Change-Id: Ibe7753e792fb6cf905bbe6815a080a8cc51c2803

(cherry picked from commit d4fb69f)

Bug: 1290858
Change-Id: Ibe7753e792fb6cf905bbe6815a080a8cc51c2803
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3414765
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#964223}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3453925
Reviewed-by: Adrian Taylor <adetaylor@google.com>
Commit-Queue: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/4758@{#1134}
Cr-Branched-From: 4a2cf4b-refs/heads/main@{#950365}

third_party/blink/renderer/core/animation/animation.cc

@@ -2320,10 +2320,6 @@ bool Animation::Update(TimingUpdateReason reason) {
 
   if (reason == kTimingUpdateForAnimationFrame) {
     if (idle || CalculateAnimationPlayState() == kFinished) {
-      // TODO(crbug.com/1029348): Per spec, we should have a microtask
-      // checkpoint right after the update cycle. Once this is fixed we should
-      // no longer need to force a synchronous resolution here.
-      AsyncFinishMicrotask();
       finished_ = true;
     }
   }

third_party/blink/renderer/core/animation/document_animations.cc

@@ -49,6 +49,7 @@
 #include "third_party/blink/renderer/core/page/page_animator.h"
 #include "third_party/blink/renderer/core/style/computed_style.h"
 #include "third_party/blink/renderer/platform/bindings/microtask.h"
+#include "third_party/blink/renderer/platform/heap/persistent.h"
 
 namespace blink {
 
@@ -260,10 +261,13 @@ void DocumentAnimations::RemoveReplacedAnimations(
 
   // The list of animations for removal is constructed in reverse composite
   // ordering for efficiency. Flip the ordering to ensure that events are
-  // dispatched in composite order.
+  // dispatched in composite order.  Queue as a microtask so that the finished
+  // event is dispatched ahead of the remove event.
   for (auto it = animations_to_remove.rbegin();
        it != animations_to_remove.rend(); it++) {
-    (*it)->RemoveReplacedAnimation();
+    Animation* animation = *it;
+    Microtask::EnqueueMicrotask(WTF::Bind(&Animation::RemoveReplacedAnimation,
+                                          WrapWeakPersistent(animation)));
   }
 }