Split audio mixer into interface and implementation.

webrtc/api/api.gyp

 # 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.
 
 {
   'includes': [ '../build/common.gypi', ],
@@ skipping 213 matching lines @@
       'type': 'static_library',
       'dependencies': [
         '<(webrtc_root)/base/base.gyp:rtc_base_approved',
       ],
       'sources': [
         'stats/rtcstats.h',
         'stats/rtcstats_objects.h',
         'stats/rtcstatsreport.h',
       ],
     },  # target rtc_stats_api
+    {
+      # GN version: webrtc/api:audio_mixer_api
+      'target_name': 'audio_mixer_api',
+      'type': 'static_library',
+      'dependencies': [
+        '<(webrtc_root)/base/base.gyp:rtc_base_approved',
+      ],
+      'sources': [
+        'audio/audio_mixer.h',
+      ],
+    },  # target rtc_stats_api
   ],  # targets
 }