Support external audio mixer.

webrtc/api/BUILD.gn

Index: webrtc/api/BUILD.gn
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index fe1ade26d61d5d96f171bef3b820144e58a0b3e5..4ee4b44ab2e495abc61f6b971fdbfb66bb9fe916 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -24,7 +24,7 @@ group("api") {
   }
 }
 
-rtc_source_set("call_api") {
+rtc_static_library("call_api") {

[GeorgeZ, 2016/12/01 17:36:32]

Based on this bug
https://buganizer.corp.google.com/u/0/issues/31847415

rtc_source-set has to change to rtc_static_library to avoid the link issue of
the application that uses webrtc.

[aleloi, 2016/12/05 14:03:08]

Please don't post internal links here.

[GeorgeZ, 2016/12/05 17:56:18]

Thanks for reminding.

On 2016/12/05 14:03:08, aleloi wrote:
Done.

   sources = [
     "call/audio_receive_stream.h",
     "call/audio_send_stream.cc",