optimizing layout performance when only transform3d matrix changed by

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
===================================================================
--- Source/core/dom/Document.h	(revision 169401)
+++ Source/core/dom/Document.h	(working copy)
@@ -1052,6 +1052,7 @@
 
     void updateStyleInvalidationIfNeeded();
 
+    void setFastCompositing(bool enable) { m_fastCompositing = enable;}
 protected:
     Document(const DocumentInit&, DocumentClassFlags = DefaultDocumentClass);
 
@@ -1353,6 +1354,8 @@
     bool m_hasViewportUnits;
 
     HashSet<DocumentVisibilityObserver*> m_visibilityObservers;
+
+    bool m_fastCompositing;
 };
 
 inline void Document::notifyRemovePendingSheetIfNeeded()