Split audio mixer into interface and implementation.

webrtc/api/api.gyp

Index: webrtc/api/api.gyp
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index d017cda2d621f18b51e05a48ba8027ca580e9a7c..374bc6541920b4867530df22526e2c2ce12c349f 100644
--- a/webrtc/api/api.gyp
+++ b/webrtc/api/api.gyp
@@ -231,5 +231,16 @@
         '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
 }