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 144 matching lines @@
     // delta and offset even though it will not be a descendant of the scroll
     // in the layer hierarchy.
     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;

[Rick Byers, 2015/01/29 20:53:13]

will we be changing this back to integer then?  If so, please add a FIXME to
make that clear.  If not, please add comments that describe the semantics of
"fractional part".  It's confusing now to think of fractions going across both
places.

     virtual WebDoublePoint scrollPositionDouble() const = 0;
+    virtual void setScrollPositionFractionalPart(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