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 7c9380c87c9f2047e37b9cc714adf48839280402..1ee77096c077994cd55123cab6c8c32c17bb8793 100644
--- a/Source/core/page/Page.cpp
+++ b/Source/core/page/Page.cpp
@@ -50,6 +50,7 @@
 #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"
@@ -223,9 +224,6 @@ ScrollingCoordinator* Page::scrollingCoordinator()
 
 String Page::mainThreadScrollingReasonsAsText()
 {
-    if (Document* document = m_mainFrame->document())
-        document->updateLayout();
-
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
         return scrollingCoordinator->mainThreadScrollingReasonsAsText();