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

Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 32 matching lines @@
 #include "core/page/FrameTree.h"
 #include "core/frame/FrameView.h"
 #include "core/page/PageConsole.h"
 #include "core/page/PageGroup.h"
 #include "core/page/PageLifecycleNotifier.h"
 #include "core/page/PointerLockController.h"
 #include "core/page/Settings.h"
 #include "core/page/ValidationMessageClient.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/plugins/PluginData.h"
+#include "core/rendering/RenderLayerCompositor.h"

[Ian Vollick, 2013/10/18 19:23:26]

We don't need this, right?

 #include "core/rendering/RenderView.h"
 #include "core/storage/StorageNamespace.h"
 #include "wtf/HashMap.h"
 #include "wtf/RefCountedLeakCounter.h"
 #include "wtf/StdLibExtras.h"
 #include "wtf/text/Base64.h"
 
 namespace WebCore {
 
 static HashSet<Page*>* allPages;
@@ skipping 153 matching lines @@
 ScrollingCoordinator* Page::scrollingCoordinator()
 {
     if (!m_scrollingCoordinator && m_settings->scrollingCoordinatorEnabled())
         m_scrollingCoordinator = ScrollingCoordinator::create(this);
 
     return m_scrollingCoordinator.get();
 }
 
 String Page::mainThreadScrollingReasonsAsText()
 {
-    if (Document* document = m_mainFrame->document())
-        document->updateLayout();
-
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
         return scrollingCoordinator->mainThreadScrollingReasonsAsText();
 
     return String();
 }
 
 PassRefPtr<ClientRectList> Page::nonFastScrollableRects(const Frame* frame)
 {
     if (Document* document = m_mainFrame->document())
         document->updateLayout();
@@ skipping 370 matching lines @@
     , inspectorClient(0)
     , backForwardClient(0)
 {
 }
 
 Page::PageClients::~PageClients()
 {
 }
 
 } // namespace WebCore