refactor EnumerateDevices to make it a persistent request.

content/common/media/media_stream_messages.h

 // Copyright (c) 2012 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.
 
 // IPC messages for the media streaming.
 // Multiply-included message file, hence no include guard.
 
 #include <string>
 
 #include "content/common/content_export.h"
@@ skipping 37 matching lines @@
 IPC_MESSAGE_ROUTED2(MediaStreamHostMsg_VideoDeviceFailed,
                     std::string /* label */,
                     int /* index */)
 
 // Report of a failure of an audio device.
 IPC_MESSAGE_ROUTED2(MediaStreamHostMsg_AudioDeviceFailed,
                     std::string /* label */,
                     int /* index */)
 
 // The browser has enumerated devices successfully.
-IPC_MESSAGE_ROUTED2(MediaStreamMsg_DevicesEnumerated,
+IPC_MESSAGE_ROUTED3(MediaStreamMsg_DevicesEnumerated,
                     int /* request id */,
+                    std::string /* label */,
                     media_stream::StreamDeviceInfoArray /* device_list */)
 
 // The browser has failed to enumerate devices.
 IPC_MESSAGE_ROUTED1(MediaStreamMsg_DevicesEnumerationFailed,
                     int /* request id */)
 
 // TODO(wjia): should DeviceOpen* messages be merged with
 // StreamGenerat* ones?
 // The browser has opened a device successfully.
 IPC_MESSAGE_ROUTED3(MediaStreamMsg_DeviceOpened,
@@ skipping 31 matching lines @@
                      media_stream::MediaStreamType /* type */,
                      GURL /* security origin */)
 
 // Request to open the device.
 IPC_MESSAGE_CONTROL5(MediaStreamHostMsg_OpenDevice,
                      int /* render view id */,
                      int /* request id */,
                      std::string /* device_id */,
                      media_stream::MediaStreamType /* type */,
                      GURL /* security origin */)