Mojom: Blanket change to mark all pointer/handle fields as nullable.

mojo/services/public/interfaces/native_viewport/native_viewport.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/gles2/command_buffer.mojom"
 import "mojo/services/public/interfaces/geometry/geometry.mojom"
 import "mojo/services/public/interfaces/input_events/input_events.mojom"
 
 module mojo {
 
 [Client=NativeViewportClient]
 interface NativeViewport {
-  Create(Rect bounds);
+  Create(Rect? bounds);
   Show();
   Hide();
   Close();
-  SetBounds(Rect bounds);
+  SetBounds(Rect? bounds);
 };
 
 interface NativeViewportClient {
   OnCreated(uint64 native_viewport_id);
-  OnBoundsChanged(Rect bounds);
+  OnBoundsChanged(Rect? bounds);
   OnDestroyed();
-  OnEvent(Event event) => ();
+  OnEvent(Event? event) => ();
 };
 
 }