Add a WebLayer:setScrollCompensationAdjustment between Blink and CC

public/platform/WebLayer.h

Index: public/platform/WebLayer.h
diff --git a/public/platform/WebLayer.h b/public/platform/WebLayer.h
index 74bf4978e4e0f83a0095f5f457519a45f76952b6..f5d9c57cca45de108c2c452a41a87966fd7a2c84 100644
--- a/public/platform/WebLayer.h
+++ b/public/platform/WebLayer.h
@@ -164,6 +164,7 @@ public:
     // 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;