Support external audio mixer in webrtc.

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

BUG=webrtc:6457

Review-Url: https://codereview.webrtc.org/2539213003
Cr-Commit-Position: refs/heads/master@{#15556}

webrtc/media/engine/webrtcmediaengine_unittest.cc

@@ -188,7 +188,8 @@ TEST(WebRtcMediaEngineFactoryTest, CreateOldApi) {
 
 TEST(WebRtcMediaEngineFactoryTest, CreateWithBuiltinDecoders) {
   std::unique_ptr<MediaEngineInterface> engine(WebRtcMediaEngineFactory::Create(
-      nullptr, webrtc::CreateBuiltinAudioDecoderFactory(), nullptr, nullptr));
+      nullptr, webrtc::CreateBuiltinAudioDecoderFactory(), nullptr, nullptr,
+      nullptr));
   EXPECT_TRUE(engine);
 }