Created an AudioMixer mock in webrtc/api/test.

webrtc/api/test/mock_audio_mixer.h

Index: webrtc/api/test/mock_audio_mixer.h
diff --git a/webrtc/api/test/mock_audio_mixer.h b/webrtc/api/test/mock_audio_mixer.h
new file mode 100644
index 0000000000000000000000000000000000000000..189455ff8a66d217c6ad1074869088bcc9e9a9e3
--- /dev/null
+++ b/webrtc/api/test/mock_audio_mixer.h
@@ -0,0 +1,32 @@
+/*
+ *  Copyright (c) 2016 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.
+ */
+
+#ifndef WEBRTC_API_TEST_MOCK_AUDIO_MIXER_H_
+#define WEBRTC_API_TEST_MOCK_AUDIO_MIXER_H_
+
+#include "webrtc/api/audio/audio_mixer.h"
+
+#include "webrtc/test/gmock.h"
+
+namespace webrtc {
+namespace test {
+
+class MockAudioMixer : public AudioMixer {
+ public:
+  MOCK_METHOD1(AddSource, bool(Source* audio_source));
+  MOCK_METHOD1(RemoveSource, void(Source* audio_source));
+  MOCK_METHOD2(Mix,
+               void(size_t number_of_channels,
+                    AudioFrame* audio_frame_for_mixing));
+};
+}  // namespace test
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_TEST_MOCK_AUDIO_MIXER_H_