Defer the real work in updateCompositingLayers until it's really needed.

Source/core/testing/Internals.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 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.
@@ skipping 281 matching lines @@
 
     String getImageSourceURL(Element*, ExceptionState&);
 
     bool isSelectPopupVisible(Node*);
 
     PassRefPtr<ClientRect> selectionBounds(ExceptionState&);
     String baseURL(Document*, ExceptionState&);
 
     bool loseSharedGraphicsContext3D();
 
+    void forceCompositingUpdate(Document*, ExceptionState&);
+
 private:
     explicit Internals(Document*);
     Document* contextDocument() const;
     Frame* frame() const;
     Vector<String> iconURLs(Document*, int iconTypesMask) const;
     PassRefPtr<ClientRectList> annotatedRegions(Document*, bool draggable, ExceptionState&);
 
     DocumentMarker* markerAt(Node*, const String& markerType, unsigned index, ExceptionState&);
     RefPtr<DOMWindow> m_frontendWindow;
     OwnPtr<InspectorFrontendChannelDummy> m_frontendChannel;
     RefPtr<InternalRuntimeFlags> m_runtimeFlags;
     RefPtr<ScrollingCoordinator> m_scrollingCoordinator;
     RefPtr<InternalProfilers> m_profilers;
 };
 
 } // namespace WebCore
 
 #endif