Mojo C++ bindings: support enum validation.

components/arc/common/power.mojom

 // Copyright 2015 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.
 
 module arc;
 
 // Enumerates the types of wake lock the ARC instance can request from the
 // host.
+[Extensible=True]
 enum DisplayWakeLockType {
   // Does not allow the screen to dim, turn off, or lock; prevents
   // idle suspend.
   BRIGHT = 0,
 
   // Allows dimming the screen, but prevents it from turning off or locking.
   // Also prevents idle suspend.
   DIM = 1
 };
 
 interface PowerHost {
   // Acquire and release wake locks on the host side.
   OnAcquireDisplayWakeLock(DisplayWakeLockType type);
   OnReleaseDisplayWakeLock(DisplayWakeLockType type);
 };
 
 interface PowerInstance {
   // Establishes full-duplex communication with the host.
   Init(PowerHost host_ptr);
 };