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

Source/core/page/Page.cpp

Index: Source/core/page/Page.cpp
diff --git a/Source/core/page/Page.cpp b/Source/core/page/Page.cpp
index 3f1af06fe405ef39f1203de46ff66e59eca3d7c5..327d89518dcd8a3beca926b0cc8872450ab64141 100644
--- a/Source/core/page/Page.cpp
+++ b/Source/core/page/Page.cpp
@@ -51,6 +51,7 @@
 #include "core/page/ValidationMessageClient.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/plugins/PluginData.h"
+#include "core/rendering/RenderLayerCompositor.h"
 #include "core/rendering/RenderView.h"
 #include "core/storage/StorageNamespace.h"
 #include "wtf/HashMap.h"
@@ -227,6 +228,9 @@ String Page::mainThreadScrollingReasonsAsText()
     if (Document* document = m_mainFrame->document())
         document->updateLayout();
 
+    if (m_mainFrame->contentRenderer() && m_mainFrame->contentRenderer()->compositor())
+        m_mainFrame->contentRenderer()->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
+
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
         return scrollingCoordinator->mainThreadScrollingReasonsAsText();