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

Source/core/testing/Internals.cpp

 /*
  * 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 1840 matching lines @@
     return String();
 }
 
 String Internals::mainThreadScrollingReasons(Document* document, ExceptionState& es) const
 {
     if (!document || !document->frame()) {
         es.throwUninformativeAndGenericDOMException(InvalidAccessError);
         return String();
     }
 
+    // Force a re-layout and a compositing update.
+    document->updateLayout();
+    RenderView* view = document->renderView();
+    if (view->compositor())
+        view->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
+
     Page* page = document->page();
     if (!page)
         return String();
 
     return page->mainThreadScrollingReasonsAsText();
 }
 
 PassRefPtr<ClientRectList> Internals::nonFastScrollableRects(Document* document, ExceptionState& es) const
 {
     if (!document || !document->frame()) {
@@ skipping 413 matching lines @@
     if (!sharedContext)
         return false;
     sharedContext->extensions()->loseContextCHROMIUM(Extensions3D::GUILTY_CONTEXT_RESET_ARB, Extensions3D::INNOCENT_CONTEXT_RESET_ARB);
     // To prevent tests that call loseSharedGraphicsContext3D from being
     // flaky, we call finish so that the context is guaranteed to be lost
     // synchronously (i.e. before returning).
     sharedContext->finish();
     return true;
 }
 
+void Internals::forceCompositingUpdate(Document* document, ExceptionState& es)
+{
+    if (!document || !document->renderView()) {
+        es.throwUninformativeAndGenericDOMException(InvalidAccessError);
+        return;
+    }
+
+    document->updateLayout();
+
+    RenderView* view = document->renderView();
+    if (view->compositor())
+        view->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
 }
+
+}