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 47 matching lines @@
 
 bool CompositorPendingAnimations::update(bool startOnCompositor)
 {
     HeapVector<Member<Animation>> waitingForStartTime;
     bool startedSynchronizedOnCompositor = false;
 
     HeapVector<Member<Animation>> animations;
     HeapVector<Member<Animation>> deferred;
     animations.swap(m_pending);
     int compositorGroup = ++m_compositorGroup;
-    if (compositorGroup == 0) {
-        // Wrap around, skipping 0.
+    while (compositorGroup == 0 || compositorGroup == 1) {

[alancutter (OOO until 2018), 2015/09/16 05:35:30]

Nit: We should use an enum instead of magic numbers.

+        // Wrap around, skipping 0, 1.
+        // * 0 is reserved for automatic assignment
+        // * 1 is used for animations with a specified start time
         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() ? 1 : compositorGroup, startOnCompositor)) {
             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