Created an AudioMixer mock in webrtc/api/test.

webrtc/api/BUILD.gn

 # 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.
 
 import("../build/webrtc.gni")
 if (is_android) {
@@ skipping 447 matching lines @@
       "//testing/gmock",
     ]
 
     if (is_android) {
       deps += [ "//testing/android/native_test:native_test_support" ]
 
       shard_timeout = 900
     }
   }
 
+  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"
       android_manifest = "androidtests/AndroidManifest.xml"
 
       java_files = [
         "androidtests/src/org/webrtc/Camera1CapturerUsingByteBufferTest.java",
         "androidtests/src/org/webrtc/Camera1CapturerUsingTextureTest.java",
         "androidtests/src/org/webrtc/Camera2CapturerTest.java",
         "androidtests/src/org/webrtc/CameraVideoCapturerTestFixtures.java",
@@ skipping 11 matching lines @@
       deps = [
         ":libjingle_peerconnection_java",
         "../base:base_java",
         "//base:base_java",
       ]
 
       shared_libraries = [ ":libjingle_peerconnection_so" ]
     }
   }
 }