Unify the calculation of main thread offset of sticky element

third_party/WebKit/public/platform/WebLayer.h

Index: third_party/WebKit/public/platform/WebLayer.h
diff --git a/third_party/WebKit/public/platform/WebLayer.h b/third_party/WebKit/public/platform/WebLayer.h
index dc6efa8392b12f1f48e546b9196107ae7ff2c404..c794c0a1ded772b86d6309f1fdcc8227136fd6ed 100644
--- a/third_party/WebKit/public/platform/WebLayer.h
+++ b/third_party/WebKit/public/platform/WebLayer.h
@@ -202,6 +202,12 @@ class WebLayer {
       const WebLayerStickyPositionConstraint&) = 0;
   virtual WebLayerStickyPositionConstraint StickyPositionConstraint() const = 0;
 
+  // Sets the relative position of the sticky box and its original position
+  // before scroll. This will be used to calculate the actual sticky position
+  // offset from cc side.
+  virtual void SetOffsetForStickyPosition(const WebSize&) = 0;
+  virtual WebSize OffsetForStickyPosition() const = 0;
+
   // The scroll client is notified when the scroll position of the WebLayer
   // changes. Only a single scroll client can be set for a WebLayer at a time.
   // The WebLayer does not take ownership of the scroll client, and it is the