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

content/common/input/web_touch_event_traits.cc

Index: content/common/input/web_touch_event_traits.cc
diff --git a/content/common/input/web_touch_event_traits.cc b/content/common/input/web_touch_event_traits.cc
index 175bf78ef41d8eb20457b8e874ec195845be86fb..32dc326d7249c8adb371f5cad83a159665251d55 100644
--- a/content/common/input/web_touch_event_traits.cc
+++ b/content/common/input/web_touch_event_traits.cc
@@ -27,7 +27,8 @@ bool WebTouchEventTraits::AllTouchPointsHaveState(
 }
 
 bool WebTouchEventTraits::IsTouchSequenceStart(const WebTouchEvent& event) {
-  DCHECK(event.touchesLength);
+  DCHECK(event.touchesLength ||
+         event.type == WebInputEvent::TouchScrollStarted);
   if (event.type != WebInputEvent::TouchStart)
     return false;
   return AllTouchPointsHaveState(event, blink::WebTouchPoint::StatePressed);