Use explicit flush for ArcVideoAccelerator.

components/arc/common/video.mojom

 // Copyright 2016 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.mojom;
 
 interface VideoHost {
   // Requests an ipc channel from Chrome to be used in the video accelerator.
   // Replies with |channel_handle| of mojo initial message pipe and |token| for
   // creating the client end of VideoAcceleratorService message pipe.
   // Replys invalid handle and empty token if any failure.
+  [MinVersion=1]
   OnRequestArcVideoAcceleratorChannel() => (handle channel_handle,
                                             string token);
 };
 
 interface VideoInstance {
   // Establishes full-duplex communication with the host.
   Init(VideoHost host_ptr);
 };