Web Animations: Plumb compositor group id through animation creation and start/finish notifications

Source/platform/graphics/GraphicsLayerClient.h

 /*
  * Copyright (C) 2009 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 42 matching lines @@
     LayerTreeIncludesPaintingPhases = 1 << 2,
     LayerTreeIncludesRootLayer = 1 << 3,
     LayerTreeIncludesClipAndScrollParents = 1 << 4
 };
 typedef unsigned LayerTreeFlags;
 
 class PLATFORM_EXPORT GraphicsLayerClient {
 public:
     virtual ~GraphicsLayerClient() {}
 
-    // Callback for when hardware-accelerated animation started.
-    virtual void notifyAnimationStarted(const GraphicsLayer*, double monotonicTime) = 0;
+    // Callback for when compositor animation started.
+    virtual void notifyAnimationStarted(const GraphicsLayer*, double monotonicTime, int group) { };
 
     virtual void paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect& inClip) = 0;
     virtual bool isTrackingPaintInvalidations() const { return false; }
 
     virtual String debugName(const GraphicsLayer*) = 0;
 
 #if ENABLE(ASSERT)
     // CompositedLayerMapping overrides this to verify that it is not
     // currently painting contents. An ASSERT fails, if it is.
     // This is executed in GraphicsLayer construction and destruction
     // to verify that we don't create or destroy GraphicsLayers
     // while painting.
     virtual void verifyNotPainting() { }
 #endif
 };
 
 } // namespace blink
 
 #endif // GraphicsLayerClient_h