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..4aea0977fffebb3b3cdef505678427c6ff0c069e 100644
--- a/third_party/WebKit/public/platform/WebLayer.h
+++ b/third_party/WebKit/public/platform/WebLayer.h
@@ -202,6 +202,11 @@ class WebLayer {
       const WebLayerStickyPositionConstraint&) = 0;
   virtual WebLayerStickyPositionConstraint StickyPositionConstraint() const = 0;
 
+  // Sets the main thread offset applied to the sticky element. This will be

[chrishtr, 2017/06/07 19:28:38]

The "main thread offset" is the of the sticky element from its non-scrolled
position right? IOW the offset from where it would be if it was
position: relative?

This is the same as GetOffsetRelativeToOriginalLayoutPosition. How about calling
the concept in cc the same? "main thread" is not necessary, because Layer is
always on the main thread and is updated only by Blink. LayerImpl is on the
impl thread and so needs to differentiate.

[yigu, 2017/06/07 21:09:14]

Done.

+  // used to calculate the actual sticky position offset from cc side.
+  virtual void SetStickyMainThreadOffset(const WebSize&) = 0;
+  virtual WebSize StickyMainThreadOffset() 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