Add pre finalizer to Animation to ensure compositor handles get cleaned up

third_party/WebKit/Source/core/animation/AnimationTimeline.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 55 matching lines @@
 }
 
 AnimationTimeline::AnimationTimeline(Document* document, PlatformTiming* timing)
     : m_document(document)
     , m_zeroTime(0) // 0 is used by unit tests which cannot initialize from the loader
     , m_zeroTimeInitialized(false)
     , m_outdatedAnimationCount(0)
     , m_playbackRate(1)
     , m_lastCurrentTimeInternal(0)
 {
-    ThreadState::current()->registerPreFinalizer(this);
     if (!timing)
         m_timing = new AnimationTimelineTiming(this);
     else
         m_timing = timing;
 
     if (Platform::current()->isThreadedAnimationEnabled())
         m_compositorTimeline = CompositorAnimationTimeline::create();
 
     ASSERT(document);
 }
 
 AnimationTimeline::~AnimationTimeline()
 {
-}
-
-void AnimationTimeline::dispose()
-{
     // The Animation objects depend on using this AnimationTimeline to
-    // unregister from its underlying compositor timeline. To arrange
-    // for that safely, this dispose() method will return first
-    // during prefinalization, notifying each Animation object of
-    // impending destruction.
+    // unregister from its underlying compositor timeline.
     for (const auto& animation : m_animations)
         animation->dispose();

[haraken, 2016/07/28 12:04:26]

I was assuming that we can entirely stop calling animation->dispose() because
now Animation objects dispose itself in their pre-finalizer.

If my understanding is wrong, we need to keep AnimationTimeline's pre-finalizer
as is because AnimationTimeline's destructor is not allowed to touch other
on-heap objects (i.e., m_animations).

 }
 
 bool AnimationTimeline::isActive()
 {
     return m_document && m_document->page();
 }
 
 void AnimationTimeline::animationAttached(Animation& animation)
 {
     ASSERT(animation.timeline() == this);
@@ skipping 244 matching lines @@
 
 DEFINE_TRACE(AnimationTimeline)
 {
     visitor->trace(m_document);
     visitor->trace(m_timing);
     visitor->trace(m_animationsNeedingUpdate);
     visitor->trace(m_animations);
 }
 
 } // namespace blink