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

content/renderer/input/input_handler_wrapper.h

Index: content/renderer/input/input_handler_wrapper.h
diff --git a/content/renderer/input/input_handler_wrapper.h b/content/renderer/input/input_handler_wrapper.h
index b32410d881415421d29cb16ee58aa811c44d5af1..7254c685027c2df4aa06ade5ef6942011c567fdd 100644
--- a/content/renderer/input/input_handler_wrapper.h
+++ b/content/renderer/input/input_handler_wrapper.h
@@ -55,6 +55,8 @@ class InputHandlerWrapper : public ui::InputHandlerProxyClient {
                      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_;