Forward GestureFlingStart in OOPIF, re-enable DCHECK.

content/browser/frame_host/render_widget_host_view_child_frame.cc

Index: content/browser/frame_host/render_widget_host_view_child_frame.cc
diff --git a/content/browser/frame_host/render_widget_host_view_child_frame.cc b/content/browser/frame_host/render_widget_host_view_child_frame.cc
index 263113a44902c9f2db46367887720c80c1fba973..8c14e80cb67245f110b24e0d70c72d64760eeeb3 100644
--- a/content/browser/frame_host/render_widget_host_view_child_frame.cc
+++ b/content/browser/frame_host/render_widget_host_view_child_frame.cc
@@ -343,8 +343,10 @@ void RenderWidgetHostViewChildFrame::GestureEventAck(
     return;
   if ((event.GetType() == blink::WebInputEvent::kGestureScrollUpdate &&
        not_consumed) ||
-      event.GetType() == blink::WebInputEvent::kGestureScrollEnd)
+      event.GetType() == blink::WebInputEvent::kGestureScrollEnd ||
+      event.GetType() == blink::WebInputEvent::kGestureFlingStart) {
     frame_connector_->BubbleScrollEvent(event);
+  }
 }
 
 void RenderWidgetHostViewChildFrame::DidReceiveCompositorFrameAck(