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

Source/web/tests/ScrollingCoordinatorChromiumTest.cpp

 /*
  * Copyright (C) 2012 Google 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 85 matching lines @@
         m_webViewImpl->close();
         m_mainFrame->close();
     }
 
     void navigateTo(const std::string& url)
     {
         FrameTestHelpers::loadFrame(m_webViewImpl->mainFrame(), url);
         Platform::current()->unitTestSupport()->serveAsynchronousMockedRequests();
     }
 
+    void forceFullCompositingUpdate()
+    {
+        RenderLayerCompositor* compositor = m_webViewImpl->mainFrameImpl()->frame()->contentRenderer()->compositor();
+        compositor->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
+    }
+
     void registerMockedHttpURLLoad(const std::string& fileName)
     {
         URLTestHelpers::registerMockedURLFromBaseURL(WebString::fromUTF8(m_baseURL.c_str()), WebString::fromUTF8(fileName.c_str()));
     }
 
     WebLayer* getRootScrollLayer()
     {
         RenderLayerCompositor* compositor = m_webViewImpl->mainFrameImpl()->frame()->contentRenderer()->compositor();
         ASSERT(compositor);
         ASSERT(compositor->scrollLayer());
@@ skipping 43 matching lines @@
     GraphicsLayer* graphicsLayer = compositedLayerMapping->mainGraphicsLayer();
     if (!graphicsLayer)
         return 0;
     return graphicsLayer->platformLayer();
 }
 
 TEST_F(ScrollingCoordinatorChromiumTest, fastScrollingForFixedPosition)
 {
     registerMockedHttpURLLoad("fixed-position.html");
     navigateTo(m_baseURL + "fixed-position.html");
+    forceFullCompositingUpdate();
 
     // Fixed position should not fall back to main thread scrolling.
     WebLayer* rootScrollLayer = getRootScrollLayer();
     ASSERT_FALSE(rootScrollLayer->shouldScrollOnMainThread());
 
     Document* document = m_webViewImpl->mainFrameImpl()->frame()->document();
     {
         Element* element = document->getElementById("div-tl");
         ASSERT_TRUE(element);
         WebLayer* layer = webLayerFromElement(element);
@@ skipping 94 matching lines @@
     navigateTo(m_baseURL + "clipped-body.html");
 
     WebLayer* rootScrollLayer = getRootScrollLayer();
     ASSERT_EQ(0u, rootScrollLayer->nonFastScrollableRegion().size());
 }
 
 TEST_F(ScrollingCoordinatorChromiumTest, overflowScrolling)
 {
     registerMockedHttpURLLoad("overflow-scrolling.html");
     navigateTo(m_baseURL + "overflow-scrolling.html");
+    forceFullCompositingUpdate();
 
     // Verify the properties of the accelerated scrolling element starting from the RenderObject
     // all the way to the WebLayer.
     Element* scrollableElement = m_webViewImpl->mainFrameImpl()->frame()->document()->getElementById("scrollable");
     ASSERT(scrollableElement);
 
     RenderObject* renderer = scrollableElement->renderer();
     ASSERT_TRUE(renderer->isBoxModelObject());
     ASSERT_TRUE(renderer->hasLayer());
 
@@ skipping 104 matching lines @@
 
 TEST_F(ScrollingCoordinatorChromiumTest, setupScrollbarLayerShouldNotCrash)
 {
     registerMockedHttpURLLoad("setup_scrollbar_layer_crash.html");
     navigateTo(m_baseURL + "setup_scrollbar_layer_crash.html");
     // This test document setup an iframe with scrollbars, then switch to
     // an empty document by javascript.
 }
 
 } // namespace