Split audio mixer into interface and implementation.

webrtc/api/BUILD.gn

Index: webrtc/api/BUILD.gn
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index bd701790b288c0b7c69fb97a525deee5417bc0ff..d671c69add059adf16c7deffd3dc902bc8ed1c0f 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -316,6 +316,17 @@ rtc_source_set("rtc_stats_api") {
   ]
 }
 
+# GYP version: webrtc/api/api.gyp:audio_mixer_api
+rtc_source_set("audio_mixer_api") {
+  sources = [
+    "audio/audio_mixer.h",

[the sun, 2016/10/12 15:26:10]

The mixer has a dependency on AudioFrame, which is in module_common_types.h.
That should probably be added here as well, otherwise the dependency chain isn't
complete?

In fact, you should probably make a CL before this one, which pulls AudioFrame
out and puts it in api/audio/ - that will be helpful for moving e.g.
AudioProcessing into api/ as well.

[aleloi, 2016/10/12 15:36:02]

There is no GN target yet for module_common_types.h. I can make one for
AudioFrame as you suggested and put in in api/audio!

[the sun, 2016/10/13 07:55:00]

I think what you need to do is pull AudioFrame out from module_common_types.h
into its own .h, put that in api/audio/ and make a target for it. But it's ok if
you do it in a follow-up.

+  ]
+
+  deps = [
+    "../base:rtc_base_approved",
+  ]
+}
+
 if (rtc_include_tests) {
   config("peerconnection_unittests_config") {
     # The warnings below are enabled by default. Since GN orders compiler flags