gesture event filter: Add a check to help debug a crash.

content/browser/renderer_host/gesture_event_filter.cc

Index: content/browser/renderer_host/gesture_event_filter.cc
diff --git a/content/browser/renderer_host/gesture_event_filter.cc b/content/browser/renderer_host/gesture_event_filter.cc
index 659decc10e48d601ff28e76133363a55f63aa122..afcde6b69c3d37226d6deb2dd83f46a8c72b61a3 100644
--- a/content/browser/renderer_host/gesture_event_filter.cc
+++ b/content/browser/renderer_host/gesture_event_filter.cc
@@ -207,6 +207,7 @@ void GestureEventFilter::Reset() {
 }
 
 void GestureEventFilter::ProcessGestureAck(bool processed, int type) {
+  CHECK(!coalesced_gesture_events_.empty());
   DCHECK_EQ(coalesced_gesture_events_.front().type, type);
   coalesced_gesture_events_.pop_front();
   if (type == WebInputEvent::GestureFlingCancel)