Remove frame flattening support as Chromium has no intention of using it

Source/core/page/FrameView.h

Index: Source/core/page/FrameView.h
diff --git a/Source/core/page/FrameView.h b/Source/core/page/FrameView.h
index bc454775ad652fb987d4687c01bac34d68b4e666..d628af11de310b090e8d8268f6379c786dbf6ac1 100644
--- a/Source/core/page/FrameView.h
+++ b/Source/core/page/FrameView.h
@@ -91,8 +91,6 @@ public:
 
     virtual PassRefPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
 
-    virtual bool avoidScrollbarCreation() const;
-
     virtual void setContentsSize(const IntSize&);
 
     void layout(bool allowSubtree = true);
@@ -247,8 +245,6 @@ public:
 
     Color documentBackgroundColor() const;
 
-    bool isInChildFrameWithFrameFlattening() const;
-
     static double currentPaintTimeStamp() { return sCurrentPaintTimeStamp; } // returns 0 if not painting
     
     void updateLayoutAndStyleIfNeededRecursive();
@@ -442,8 +438,6 @@ private:
 
     FrameView* parentFrameView() const;
 
-    bool doLayoutWithFrameFlattening(bool allowSubtree);
-
     virtual AXObjectCache* axObjectCache() const;
     void notifyWidgetsInAllFrames(WidgetNotification);
     void removeFromAXObjectCache();