Unify the calculation of main thread offset of sticky element

cc/blink/web_layer_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_BLINK_WEB_LAYER_IMPL_H_
 #define CC_BLINK_WEB_LAYER_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 99 matching lines @@
   blink::WebVector<blink::WebRect> TouchEventHandlerRegion() const override;
   void SetIsContainerForFixedPositionLayers(bool is_container) override;
   bool IsContainerForFixedPositionLayers() const override;
   void SetPositionConstraint(
       const blink::WebLayerPositionConstraint& constraint) override;
   blink::WebLayerPositionConstraint PositionConstraint() const override;
   void SetStickyPositionConstraint(
       const blink::WebLayerStickyPositionConstraint& constraint) override;
   blink::WebLayerStickyPositionConstraint StickyPositionConstraint()
       const override;
+  void SetOffsetForStickyPosition(const blink::WebSize& offset) override;
+  blink::WebSize OffsetForStickyPosition() const override;
   void SetScrollClient(blink::WebLayerScrollClient* client) override;
   void SetLayerClient(cc::LayerClient* client) override;
   const cc::Layer* CcLayer() const override;
   cc::Layer* CcLayer() override;
   void SetElementId(const cc::ElementId&) override;
   cc::ElementId GetElementId() const override;
   void SetCompositorMutableProperties(uint32_t properties) override;
   uint32_t CompositorMutableProperties() const override;
   void SetHasWillChangeTransformHint(bool has_will_change) override;
   void ShowScrollbars() override;
 
   void SetScrollParent(blink::WebLayer* parent) override;
   void SetClipParent(blink::WebLayer* parent) override;
 
  protected:
   scoped_refptr<cc::Layer> layer_;
 
   bool contents_opaque_is_fixed_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WebLayerImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_LAYER_IMPL_H_