Fix smooth scroll jump when switching scroll handling between MT and CC

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 261 matching lines @@
   void SetScrollClipLayerId(int clip_layer_id);
   bool scrollable() const { return scroll_clip_layer_id_ != INVALID_ID; }
 
   void SetUserScrollable(bool horizontal, bool vertical);
   bool user_scrollable_horizontal() const {
     return user_scrollable_horizontal_;
   }
   bool user_scrollable_vertical() const { return user_scrollable_vertical_; }
 
   void AddMainThreadScrollingReasons(uint32_t main_thread_scrolling_reasons);
+  void ClearMainThreadScrollingReasons(
+      uint32_t main_thread_scrolling_reasons_to_clear);
   void ClearMainThreadScrollingReasons();
   uint32_t main_thread_scrolling_reasons() const {
     return main_thread_scrolling_reasons_;
   }
   bool should_scroll_on_main_thread() const {
     return !!main_thread_scrolling_reasons_;
   }
 
   void SetNonFastScrollableRegion(const Region& non_fast_scrollable_region);
   const Region& non_fast_scrollable_region() const {
@@ skipping 488 matching lines @@
 
   std::vector<FrameTimingRequest> frame_timing_requests_;
   bool frame_timing_requests_dirty_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_