Don't update graphics layer positions during coordinated scrolling

Source/core/page/scrolling/ScrollingCoordinator.h

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 72 matching lines @@
     enum MainThreadScrollingReasonFlags {
         HasSlowRepaintObjects = 1 << 0,
         HasViewportConstrainedObjectsWithoutSupportingFixedLayers = 1 << 1,
         HasNonLayerViewportConstrainedObjects = 1 << 2,
     };
 
     MainThreadScrollingReasons mainThreadScrollingReasons() const;
     bool shouldUpdateScrollLayerPositionOnMainThread() const { return mainThreadScrollingReasons() != 0; }
 
     void willDestroyScrollableArea(ScrollableArea*);
-    void scrollableAreaScrollLayerDidChange(ScrollableArea*);
+    // Returns true iff the coordinator handled this change.

[jamesr, 2013/07/09 23:39:38]

while iff is more precise than if, i don't think it's used very widely in blink
compared to 'if' so we should probably just use that. just don't tell my
geometry teacher!

[enne (OOO), 2013/07/09 23:43:47]

Iff you say so.  Done.

+    bool scrollableAreaScrollLayerDidChange(ScrollableArea*);
     void scrollableAreaScrollbarLayerDidChange(ScrollableArea*, ScrollbarOrientation);
     void setLayerIsContainerForFixedPositionLayers(GraphicsLayer*, bool);
     void updateLayerPositionConstraint(RenderLayer*);
     void touchEventTargetRectsDidChange(const Document*);
 
     void computeAbsoluteTouchEventTargetRects(const Document*, Vector<IntRect>&);
 
     static String mainThreadScrollingReasonsAsText(MainThreadScrollingReasons);
     String mainThreadScrollingReasonsAsText() const;
     Region computeShouldHandleScrollGestureOnMainThreadRegion(const Frame*, const IntPoint& frameLocation) const;
@@ skipping 30 matching lines @@
 
     typedef HashMap<ScrollableArea*, OwnPtr<WebKit::WebScrollbarLayer> > ScrollbarMap;
     ScrollbarMap m_horizontalScrollbars;
     ScrollbarMap m_verticalScrollbars;
 
 };
 
 } // namespace WebCore
 
 #endif // ScrollingCoordinator_h