Send a GSB to main thread before switching to it in the middle of scrolling.

content/renderer/input/input_handler_wrapper.h

 // Copyright 2013 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 CONTENT_RENDERER_INPUT_INPUT_HANDLER_WRAPPER_H_
 #define CONTENT_RENDERER_INPUT_INPUT_HANDLER_WRAPPER_H_
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 37 matching lines @@
   blink::WebGestureCurve* CreateFlingAnimationCurve(
       blink::WebGestureDevice deviceSource,
       const blink::WebFloatPoint& velocity,
       const blink::WebSize& cumulativeScroll) override;
   void DidOverscroll(const gfx::Vector2dF& accumulated_overscroll,
                      const gfx::Vector2dF& latest_overscroll_delta,
                      const gfx::Vector2dF& current_fling_velocity,
                      const gfx::PointF& causal_event_viewport_point) override;
   void DidStopFlinging() override;
   void DidAnimateForInput() override;
+  void GenerateScrollBeginAndSendToMainThread(
+      const blink::WebGestureEvent& update_event) override;
 
  private:
   InputHandlerManager* input_handler_manager_;
   int routing_id_;
   ui::InputHandlerProxy input_handler_proxy_;
   scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
 
   // Can only be accessed on the main thread.
   base::WeakPtr<RenderWidget> render_widget_;
 
   DISALLOW_COPY_AND_ASSIGN(InputHandlerWrapper);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_INPUT_INPUT_HANDLER_WRAPPER_H_