Web Animations: Don't participate in grouping when start time is set

Source/core/animation/CompositorPendingAnimations.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 54 matching lines @@
     HeapVector<Member<Animation>> deferred;
     animations.swap(m_pending);
     int compositorGroup = ++m_compositorGroup;
     if (compositorGroup == 0) {
         // Wrap around, skipping 0.
         compositorGroup = ++m_compositorGroup;
     }
 
     for (auto& animation : animations) {
         bool hadCompositorAnimation = animation->hasActiveAnimationsOnCompositor();
-        if (animation->preCommit(compositorGroup, startOnCompositor)) {
+        // Animations with a start time do not participate in compositor start-time grouping.
+        if (animation->preCommit(animation->hasStartTime() ? 0 : compositorGroup, startOnCompositor)) {

[ctserng, 2015/09/14 14:30:32]

Will we run the risk of hitting the assert here:

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

             if (animation->hasActiveAnimationsOnCompositor() && !hadCompositorAnimation) {
                 startedSynchronizedOnCompositor = true;
             }
 
             if (animation->playing() && !animation->hasStartTime()) {
                 waitingForStartTime.append(animation.get());
             }
         } else {
             deferred.append(animation);
         }
@@ skipping 67 matching lines @@
 
 }
 
 DEFINE_TRACE(CompositorPendingAnimations)
 {
     visitor->trace(m_pending);
     visitor->trace(m_waitingForCompositorAnimationStart);
 }
 
 } // namespace