Split audio mixer into interface and implementation.

webrtc/modules/audio_mixer/audio_mixer.gypi

 # 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.
 
 {
   'targets': [
     {
-      'target_name': 'audio_mixer',
+      'target_name': 'audio_mixer_impl',
       'type': 'static_library',
       'dependencies': [
+        'audio_frame_manipulator',
         'audio_processing',
         'webrtc_utility',
+        '<(webrtc_root)/api/api.gyp:audio_mixer_api',
+        '<(webrtc_root)/base/base.gyp:rtc_base_approved',
         '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
-        '<(webrtc_root)/base/base.gyp:rtc_base_approved',
-        '<(webrtc_root)/voice_engine/voice_engine.gyp:level_indicator',
       ],
       'sources': [
-        'audio_frame_manipulator.cc',
-        'audio_frame_manipulator.h',
-        'audio_mixer.h',
         'audio_mixer_impl.cc',
         'audio_mixer_impl.h',
         'audio_source_with_mix_status.cc',
         'audio_source_with_mix_status.h',
       ],
     },
+    {
+      'target_name': 'audio_frame_manipulator',
+      'type': 'static_library',
+      'dependencies': [
+          'webrtc_utility',
+          '<(webrtc_root)/base/base.gyp:rtc_base_approved',
+      ],
+      'sources': [
+        'audio_frame_manipulator.cc',
+        'audio_frame_manipulator.h',
+      ],
+    },
   ], # targets
 }