Mojom cpp bindings: Enable non-nullable check.

mojo/services/public/interfaces/input_events/input_events.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import "mojo/services/public/interfaces/geometry/geometry.mojom"
 import "mojo/services/public/interfaces/input_events/input_event_constants.mojom"
 
 module mojo {
 
 struct LocationData {
@@ skipping 12 matching lines @@
 
 struct MouseWheelData {
   int32 x_offset;
   int32 y_offset;
 };
 
 struct Event {
   EventType action;
   EventFlags flags;
   int64 time_stamp;
-  LocationData location_data;
-  KeyData key_data;
-  TouchData touch_data;
-  MouseWheelData wheel_data;
+  LocationData? location_data;
+  KeyData? key_data;
+  TouchData? touch_data;
+  MouseWheelData? wheel_data;
 };
 
 }