The GestureRecognizer should never map an ID to a NULL consumer

ui/events/gestures/gesture_recognizer_impl.cc

Index: ui/events/gestures/gesture_recognizer_impl.cc
diff --git a/ui/events/gestures/gesture_recognizer_impl.cc b/ui/events/gestures/gesture_recognizer_impl.cc
index 950bec9b3a93e8378aac99a092a457f016532f8c..acd964305897a83cd34598cc3c90f005ea282013 100644
--- a/ui/events/gestures/gesture_recognizer_impl.cc
+++ b/ui/events/gestures/gesture_recognizer_impl.cc
@@ -83,10 +83,13 @@ GestureConsumer* GestureRecognizerImpl::GetTouchLockedTarget(
 
 GestureConsumer* GestureRecognizerImpl::GetTargetForGestureEvent(
     const GestureEvent& event) {
-  GestureConsumer* target = NULL;
   int touch_id = event.details().oldest_touch_id();
-  target = touch_id_target_for_gestures_[touch_id];
-  return target;
+  if (!touch_id_target_for_gestures_.count(touch_id)) {
+    NOTREACHED() << "Touch ID does not map to a valid GestureConsumer.";
+    return nullptr;
+  }
+
+  return touch_id_target_for_gestures_.at(touch_id);
 }
 
 GestureConsumer* GestureRecognizerImpl::GetTargetForLocation(