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

Source/web/tests/ScrollingCoordinatorChromiumTest.cpp

Index: Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
diff --git a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
index dc845c34bc0a75428588bc11b1a65e251d8ab6bc..316e5bd811d012a8522b31074e0c39176693ae81 100644
--- a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
+++ b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
@@ -103,6 +103,12 @@ public:
         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()));
@@ -166,6 +172,7 @@ 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();
@@ -280,6 +287,7 @@ 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.