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

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 106 matching lines @@
   void setScrollPositionDouble(blink::WebDoublePoint position) override;
   blink::WebDoublePoint scrollPositionDouble() const override;
   void setScrollCompensationAdjustment(blink::WebDoublePoint position) override;
   void setScrollClipLayer(blink::WebLayer* clip_layer) override;
   bool scrollable() const override;
   void setUserScrollable(bool horizontal, bool vertical) override;
   bool userScrollableHorizontal() const override;
   bool userScrollableVertical() const override;
   void addMainThreadScrollingReasons(
       uint32_t main_thread_scrolling_reasons) override;
-  void clearMainThreadScrollingReasons() override;
+  void clearMainThreadScrollingReasons(
+      uint32_t main_thread_scrolling_reasons_to_clear) override;
+  uint32_t mainThreadScrollingReasons() override;
   bool shouldScrollOnMainThread() const override;
   void setNonFastScrollableRegion(
       const blink::WebVector<blink::WebRect>& region) override;
   blink::WebVector<blink::WebRect> nonFastScrollableRegion() const override;
   void setTouchEventHandlerRegion(
       const blink::WebVector<blink::WebRect>& region) override;
   blink::WebVector<blink::WebRect> touchEventHandlerRegion() const override;
   void setFrameTimingRequests(
       const blink::WebVector<std::pair<int64_t, blink::WebRect>>& requests)
       override;
@@ skipping 22 matching lines @@
 
  private:
   scoped_ptr<WebToCCAnimationDelegateAdapter> animation_delegate_adapter_;
 
   DISALLOW_COPY_AND_ASSIGN(WebLayerImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_LAYER_IMPL_H_