Support external audio mixer.

webrtc/media/engine/webrtcmediaengine_unittest.cc

 /*
  *  Copyright (c) 2015 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 170 matching lines @@
 }
 
 TEST(WebRtcMediaEngineFactoryTest, CreateOldApi) {
   std::unique_ptr<MediaEngineInterface> engine(
       WebRtcMediaEngineFactory::Create(nullptr, nullptr, nullptr));
   EXPECT_TRUE(engine);
 }
 
 TEST(WebRtcMediaEngineFactoryTest, CreateWithBuiltinDecoders) {
   std::unique_ptr<MediaEngineInterface> engine(WebRtcMediaEngineFactory::Create(
-      nullptr, webrtc::CreateBuiltinAudioDecoderFactory(), nullptr, nullptr));
+      nullptr, webrtc::CreateBuiltinAudioDecoderFactory(), nullptr, nullptr,
+      nullptr));
   EXPECT_TRUE(engine);
 }
 
 }  // namespace cricket