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

content/browser/renderer_host/render_widget_host_view_aura.cc

Index: content/browser/renderer_host/render_widget_host_view_aura.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.cc b/content/browser/renderer_host/render_widget_host_view_aura.cc
index 02820033e888a3ac38ff3874f924c3bc55c52331..4ae64acee9221dd496bebe27f3277da5d3bd2372 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -1155,6 +1155,10 @@ void RenderWidgetHostViewAura::ProcessAckedTouchEvent(
   if (!host)
     return;
 
+  // The TouchScrollStarted event is generated & consumed downstream from the
+  // TouchEventQueue. So we don't expect an ACK up here.
+  DCHECK(touch.event.type != blink::WebInputEvent::TouchScrollStarted);
+
   ui::EventResult result = (ack_result == INPUT_EVENT_ACK_STATE_CONSUMED)
                                ? ui::ER_HANDLED
                                : ui::ER_UNHANDLED;