Notify Blink about start of gesture scroll through a queued event.

content/browser/renderer_host/input/input_router_impl.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/browser/renderer_host/input/input_router_impl.h"
 
 #include <math.h>
 #include <utility>
 
 #include "base/auto_reset.h"
@@ skipping 140 matching lines @@
     const GestureEventWithLatencyInfo& original_gesture_event) {
   input_stream_validator_.Validate(original_gesture_event.event);
 
   GestureEventWithLatencyInfo gesture_event(original_gesture_event);
 
   if (touch_action_filter_.FilterGestureEvent(&gesture_event.event))
     return;
 
   wheel_event_queue_.OnGestureScrollEvent(gesture_event);
 
-  if (gesture_event.event.sourceDevice == blink::WebGestureDeviceTouchscreen)
+  if (gesture_event.event.sourceDevice == blink::WebGestureDeviceTouchscreen) {
+    if (gesture_event.event.type == blink::WebInputEvent::GestureScrollBegin)
+      touch_event_queue_.PrependTouchScrollNotification();
     touch_event_queue_.OnGestureScrollEvent(gesture_event);
+  }
 
   gesture_event_queue_.QueueEvent(gesture_event);
 }
 
 void InputRouterImpl::SendTouchEvent(
     const TouchEventWithLatencyInfo& touch_event) {
   input_stream_validator_.Validate(touch_event.event);
   touch_event_queue_.QueueEvent(touch_event);
 }
 
@@ skipping 465 matching lines @@
 
   flush_requested_ = false;
   client_->DidFlush();
 }
 
 void InputRouterImpl::SetFrameTreeNodeId(int frameTreeNodeId) {
   frame_tree_node_id_ = frameTreeNodeId;
 }
 
 }  // namespace content