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

content/renderer/input/input_handler_wrapper.cc

 // 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.
 
 #include "content/renderer/input/input_handler_wrapper.h"
 
 #include "base/command_line.h"
 #include "base/location.h"
 #include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
@@ skipping 72 matching lines @@
 }
 
 void InputHandlerWrapper::DidStopFlinging() {
   input_handler_manager_->DidStopFlinging(routing_id_);
 }
 
 void InputHandlerWrapper::DidAnimateForInput() {
   input_handler_manager_->DidAnimateForInput();
 }
 
+void InputHandlerWrapper::GenerateScrollBeginAndSendToMainThread(
+    const blink::WebGestureEvent& update_event) {
+  DCHECK_EQ(update_event.GetType(), blink::WebInputEvent::kGestureScrollUpdate);
+  blink::WebGestureEvent scroll_begin(update_event);
+  scroll_begin.SetType(blink::WebInputEvent::kGestureScrollBegin);
+  scroll_begin.data.scroll_begin.inertial_phase =
+      update_event.data.scroll_update.inertial_phase;
+  scroll_begin.data.scroll_begin.delta_x_hint =
+      update_event.data.scroll_update.delta_x;
+  scroll_begin.data.scroll_begin.delta_y_hint =
+      update_event.data.scroll_update.delta_y;
+  scroll_begin.data.scroll_begin.delta_hint_units =
+      update_event.data.scroll_update.delta_units;
+
+  DispatchNonBlockingEventToMainThread(
+      ui::WebInputEventTraits::Clone(scroll_begin), ui::LatencyInfo());
+}
+
 }  // namespace content