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

Source/platform/graphics/GraphicsLayerClient.h

Index: Source/platform/graphics/GraphicsLayerClient.h
diff --git a/Source/platform/graphics/GraphicsLayerClient.h b/Source/platform/graphics/GraphicsLayerClient.h
index 022ca3107488bfc95b2fadf193213bd4891da151..d422b585040683ba5b51f24433eb4672409e5176 100644
--- a/Source/platform/graphics/GraphicsLayerClient.h
+++ b/Source/platform/graphics/GraphicsLayerClient.h
@@ -60,8 +60,8 @@ 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; }