Add a WebLayer:setScrollCompensationAdjustment between Blink and CC

public/platform/WebLayer.h

 /*
  * Copyright (C) 2011 Google 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
@@ skipping 146 matching lines @@
     virtual void setScrollParent(WebLayer*) = 0;
 
     // A layer will not respect any clips established by layers between it and
     // its nearest clipping ancestor. Note, the clip parent must be an ancestor.
     // This is not a requirement of the scroll parent.
     virtual void setClipParent(WebLayer*) = 0;
 
     // Scrolling
     virtual void setScrollPositionDouble(WebDoublePoint) = 0;
     virtual WebDoublePoint scrollPositionDouble() const = 0;
+    // Blink tells cc the scroll offset through setScrollPositionDouble() using
+    // floating precision but it currently can only position cc layers at integer
+    // boundary. So Blink needs to also call setScrollCompensationAdjustment()
+    // to tell cc what's the part of the scroll offset that Blink doesn't handle
+    // but cc needs to take into consideration, e.g. compensating
+    // for fixed-position layer that's positioned in Blink using only integer scroll
+    // offset.
+    // We make this call explicit, instead of letting cc to infer the fractional part
+    // from the scroll offset, to be clear that this is Blink's limitation. Once
+    // Blink can fully handle fractional scroll offset, it can stop calling
+    // this function and cc side would just work.
+    virtual void setScrollCompensationAdjustment(WebDoublePoint) = 0;
 
     // To set a WebLayer as scrollable we must specify the corresponding clip layer.
     virtual void setScrollClipLayer(WebLayer*) = 0;
     virtual bool scrollable() const = 0;
     virtual void setUserScrollable(bool horizontal, bool vertical) = 0;
     virtual bool userScrollableHorizontal() const = 0;
     virtual bool userScrollableVertical() const = 0;
 
     virtual void setHaveWheelEventHandlers(bool) = 0;
     virtual bool haveWheelEventHandlers() const = 0;
@@ skipping 35 matching lines @@
 
     // True if the layer is not part of a tree attached to a WebLayerTreeView.
     virtual bool isOrphan() const = 0;
 
     virtual void setWebLayerClient(WebLayerClient*) = 0;
 };
 
 } // namespace blink
 
 #endif // WebLayer_h