Don't schedule an animation during compositing dirty-bit setting unless needed.

Source/core/rendering/compositing/RenderLayerCompositor.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 181 matching lines @@
     void addOutOfFlowPositionedLayer(RenderLayer*);
     void removeOutOfFlowPositionedLayer(RenderLayer*);
 
     void resetTrackedRepaintRects();
     void setTracksRepaints(bool);
 
     void setNeedsToRecomputeCompositingRequirements() { m_needsToRecomputeCompositingRequirements = true; }
 
     virtual String debugName(const GraphicsLayer*) OVERRIDE;
 
+    void scheduleAnimationIfNeeded();
+
 private:
     class OverlapMap;
 
     enum CompositingStateTransitionType {
         NoCompositingStateChange,
         AllocateOwnCompositedLayerMapping,
         RemoveOwnCompositedLayerMapping,
         PutInSquashingLayer,
         RemoveFromSquashingLayer
     };
@@ skipping 14 matching lines @@
         // Absolute coordinates of the compositedLayerMapping's owning layer. This is used for computing the correct
         // positions of renderlayers when they paint into the squashing layer.
         IntPoint offsetFromAbsoluteForSquashingCLM;
 
         // Counter that tracks what index the next RenderLayer would be if it gets squashed to the current squashing layer.
         size_t nextSquashedLayerIndex;
 
         RenderLayer* clippingAncestorForMostRecentMapping;
     };
 
+    bool hasUnresolvedDirtyBits();
+
     bool canSquashIntoCurrentSquashingOwner(const RenderLayer* candidate, const SquashingState&, const RenderLayer* clippingAncestor);
 
     CompositingStateTransitionType computeCompositedLayerUpdate(RenderLayer*);
     // Make updates to the layer based on viewport-constrained properties such as position:fixed. This can in turn affect
     // compositing.
     bool updateLayerIfViewportConstrained(RenderLayer*);
 
     // GraphicsLayerClient implementation
     virtual void notifyAnimationStarted(const GraphicsLayer*, double) OVERRIDE { }
     virtual void paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect&) OVERRIDE;
@@ skipping 126 matching lines @@
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h