Made MixerAudioSource a pure interface.

webrtc/modules/audio_mixer/BUILD.gn

Index: webrtc/modules/audio_mixer/BUILD.gn
diff --git a/webrtc/modules/audio_mixer/BUILD.gn b/webrtc/modules/audio_mixer/BUILD.gn
index 866dcd812f572d5e82aa343df4236fb46ce5fabe..763b6641420f6595733e82471b0978412d51eed9 100644
--- a/webrtc/modules/audio_mixer/BUILD.gn
+++ b/webrtc/modules/audio_mixer/BUILD.gn
@@ -21,6 +21,8 @@ rtc_static_library("audio_mixer") {
     "audio_mixer_defines.h",
     "audio_mixer_impl.cc",
     "audio_mixer_impl.h",
+    "audio_source_with_mix_status.cc",
+    "audio_source_with_mix_status.h",
   ]
 
   public = [
@@ -32,6 +34,7 @@ rtc_static_library("audio_mixer") {
 
   deps = [
     "../..:webrtc_common",
+    "../../base:gtest_prod",

[aleloi, 2016/10/04 13:37:15]

To make the tests friends with AudioMixerImpl::Create so that tests can access
GetAudioSourceMixabilityStatus without having to expose it to the mixer
interface.

     "../../base:rtc_base_approved",
     "../../modules/audio_processing",
     "../../modules/utility",