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 099d29f0112973af828b491cf255fe71110afd45..bc559d5b0eb12c866a42f124ecf7fd19a5430945 100644
--- a/ui/events/gestures/gesture_recognizer_impl.cc
+++ b/ui/events/gestures/gesture_recognizer_impl.cc
@@ -87,10 +87,13 @@ 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.";

[tdanderson, 2014/08/13 15:48:00]

Running this through the trybots to see if anything hits this line. We may or
may not want to include this NOTREACHED().

+    return NULL;
+  }
+
+  return touch_id_target_for_gestures_.at(touch_id);
 }
 
 GestureConsumer* GestureRecognizerImpl::GetTargetForLocation(