getUserMeida: report device starting states

services/video_capture/public/interfaces/receiver.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 video_capture.mojom;
 
 import "media/mojo/interfaces/media_types.mojom";
 
 // Callback interface for receiving data and messages from a started
 // video_capture.mojom.Device.
 interface Receiver {
   OnIncomingCapturedVideoFrame(media.mojom.VideoFrame frame);
   OnError();
   OnLog(string message);
+  OnStarted();
   OnBufferDestroyed(int32 buffer_id_to_drop);
 };