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

content/browser/renderer_host/input/input_router_impl.cc

Index: content/browser/renderer_host/input/input_router_impl.cc
diff --git a/content/browser/renderer_host/input/input_router_impl.cc b/content/browser/renderer_host/input/input_router_impl.cc
index 4f020675f6c9dede08ec957b15b5cd726d461bf5..19006e1cc5a5455fa31c6fef5383184b3029769d 100644
--- a/content/browser/renderer_host/input/input_router_impl.cc
+++ b/content/browser/renderer_host/input/input_router_impl.cc
@@ -158,8 +158,11 @@ void InputRouterImpl::SendGestureEvent(
 
   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);
 }
@@ -365,20 +368,21 @@ void InputRouterImpl::OfferToHandlers(const WebInputEvent& input_event,
     return;
 
   // Touch events should always indicate in the event whether they are
-  // cancelable (respect ACK disposition) or not except touchmove.
+  // cancelable (respect ACK disposition) or not, except touchmove and
+  // touchscollstarted.
   bool should_block = WebInputEventTraits::ShouldBlockEventStream(input_event);
+  if (WebInputEvent::isTouchEventType(input_event.type) &&
+      input_event.type != WebInputEvent::TouchMove &&
+      input_event.type != WebInputEvent::TouchScrollStarted) {
+    const WebTouchEvent& touch = static_cast<const WebTouchEvent&>(input_event);
+    DCHECK_EQ(should_block, touch.cancelable);
+  }
 
   OfferToRenderer(input_event, latency_info,
                   should_block
                       ? InputEventDispatchType::DISPATCH_TYPE_BLOCKING
                       : InputEventDispatchType::DISPATCH_TYPE_NON_BLOCKING);
 
-  if (WebInputEvent::isTouchEventType(input_event.type) &&
-      input_event.type != WebInputEvent::TouchMove) {
-    const WebTouchEvent& touch = static_cast<const WebTouchEvent&>(input_event);
-    DCHECK_EQ(should_block, touch.cancelable);
-  }
-
   // Generate a synthetic ack if the event was sent so it doesn't block.
   if (!should_block) {
     ProcessInputEventAck(