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

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

Index: third_party/WebKit/Source/core/animation/AnimationTimeline.cpp
diff --git a/third_party/WebKit/Source/core/animation/AnimationTimeline.cpp b/third_party/WebKit/Source/core/animation/AnimationTimeline.cpp
index 99b35e68c0b3892259cf7147c11cd1164876cb9d..b373e51325c3b131ade63d32212856b833916815 100644
--- a/third_party/WebKit/Source/core/animation/AnimationTimeline.cpp
+++ b/third_party/WebKit/Source/core/animation/AnimationTimeline.cpp
@@ -73,7 +73,6 @@ AnimationTimeline::AnimationTimeline(Document* document, PlatformTiming* timing)
     , m_playbackRate(1)
     , m_lastCurrentTimeInternal(0)
 {
-    ThreadState::current()->registerPreFinalizer(this);
     if (!timing)
         m_timing = new AnimationTimelineTiming(this);
     else
@@ -87,15 +86,8 @@ AnimationTimeline::AnimationTimeline(Document* document, PlatformTiming* timing)
 
 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).

 }