Allow WebRtcMediaEngine to be created from any thread.

webrtc/media/engine/webrtcmediaengine.h

 /*
  *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 14 matching lines @@
 }
 namespace cricket {
 class WebRtcVideoDecoderFactory;
 class WebRtcVideoEncoderFactory;
 }
 
 namespace cricket {
 
 class WebRtcMediaEngineFactory {
  public:
+  // These Create methods may be called on any thread, though the engine is
+  // only expected to be used on one thread, internally called the "worker
+  // thread". This is the thread Init must be called on.
+
   // TODO(ossu): Backwards-compatible interface. Will be deprecated once the
   // audio decoder factory is fully plumbed and used throughout WebRTC.
   // See: crbug.com/webrtc/6000
   static MediaEngineInterface* Create(
       webrtc::AudioDeviceModule* adm,
       WebRtcVideoEncoderFactory* video_encoder_factory,
       WebRtcVideoDecoderFactory* video_decoder_factory);
 
   // TODO(deadbeef): Change these to return an std::unique_ptr<>, to indicate
   // that the caller owns the returned object.
@@ skipping 43 matching lines @@
     const std::vector<webrtc::RtpExtension>& extensions,
     bool (*supported)(const std::string&),
     bool filter_redundant_extensions);
 
 webrtc::Call::Config::BitrateConfig GetBitrateConfigForCodec(
     const Codec& codec);
 
 }  // namespace cricket
 
 #endif  // WEBRTC_MEDIA_ENGINE_WEBRTCMEDIAENGINE_H_