Unify the calculation of main thread offset of sticky element

cc/layers/layer.h

 // Copyright 2010 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_LAYERS_LAYER_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 156 matching lines @@
   const LayerPositionConstraint& position_constraint() const {
     return inputs_.position_constraint;
   }
 
   void SetStickyPositionConstraint(
       const LayerStickyPositionConstraint& constraint);
   const LayerStickyPositionConstraint& sticky_position_constraint() const {
     return inputs_.sticky_position_constraint;
   }
 
+  void SetStickyMainThreadOffset(const gfx::Size& offset);
+  const gfx::Size sticky_main_thread_offset() const {
+    return inputs_.sticky_main_thread_offset;
+  }
+
   void SetTransform(const gfx::Transform& transform);
   const gfx::Transform& transform() const { return inputs_.transform; }
 
   void SetTransformOrigin(const gfx::Point3F&);
   gfx::Point3F transform_origin() const { return inputs_.transform_origin; }
 
   void SetScrollParent(Layer* parent);
 
   Layer* scroll_parent() { return inputs_.scroll_parent; }
 
@@ skipping 383 matching lines @@
 
     uint32_t main_thread_scrolling_reasons;
     Region non_fast_scrollable_region;
 
     Region touch_event_handler_region;
 
     bool is_container_for_fixed_position_layers : 1;
     LayerPositionConstraint position_constraint;
 
     LayerStickyPositionConstraint sticky_position_constraint;
+    gfx::Size sticky_main_thread_offset;

[trchen, 2017/06/05 22:43:11]

I think we should document what this value means. At the beginning I thought it
is accumulated offset up to the enclosing overflow clip, but it is in fact the
stick offset applied locally.

[yigu, 2017/06/06 19:33:18]

Done.

 
     ElementId element_id;
 
     uint32_t mutable_properties;
 
     Layer* scroll_parent;
     Layer* clip_parent;
 
     bool has_will_change_transform_hint : 1;
 
@@ skipping 40 matching lines @@
   // These all act like draw properties, so don't need push properties.
   gfx::Rect visible_layer_rect_;
   size_t num_unclipped_descendants_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_