Support external audio mixer in webrtc 2.

An external audio mixer will be passed from PeerConnectionFactory to
AudioTransportProxy.

This CL has rewritten based on reverted CL
https://codereview.chromium.org/2539213003/
The only difference is that
  static MediaEngineInterface* Create(
      webrtc::AudioDeviceModule* adm,
      const rtc::scoped_refptr<webrtc::AudioDecoderFactory>&
          audio_decoder_factory,
      WebRtcVideoEncoderFactory* video_encoder_factory,
      WebRtcVideoDecoderFactory* video_decoder_factory);
in media/engine/webrtcmediaengine.h is kept in this CL instead of
replaced for backward compatibility.

BUG=webrtc:6457

Review-Url: https://codereview.webrtc.org/2570993002
Cr-Commit-Position: refs/heads/master@{#15580}