Created an AudioMixer mock in webrtc/api/test.

The mock is used in a dependent CL https://codereview.webrtc.org/2436033002.

There is also a goal to allow external mixing implementations
(subclasses of webrtc::AudioMixer) and inject them to
PeerConnectionFactory. We think that part of that is an official and
maintained mock.

Summary of changes:
    * Created a mixer mock/stub in webrtc/api/test
    * Made a target webrtc/api:mock_audio_mixer for it.

NOTRY=True
BUG=webrtc:6346

Review-Url: https://codereview.webrtc.org/2520323002
Cr-Commit-Position: refs/heads/master@{#15190}

webrtc/api/BUILD.gn

@@ -465,6 +465,21 @@ if (rtc_include_tests) {
     }
   }
 
+  rtc_source_set("mock_audio_mixer") {
+    testonly = true
+    sources = [
+      "test/mock_audio_mixer.h",
+    ]
+
+    public_deps = [
+      ":audio_mixer_api",
+    ]
+
+    deps = [
+      "//testing/gmock",
+    ]
+  }
+
   if (is_android) {
     instrumentation_test_apk("libjingle_peerconnection_android_unittest") {
       apk_name = "libjingle_peerconnection_android_unittest"

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_