Split audio mixer into interface and implementation.

webrtc/modules/BUILD.gn

Index: webrtc/modules/BUILD.gn
diff --git a/webrtc/modules/BUILD.gn b/webrtc/modules/BUILD.gn
index ff0918e6d4d7c8b06cc15251aa6d580d23297b7b..089bbbfc287882d77debedf0572f6593ebc27577 100644
--- a/webrtc/modules/BUILD.gn
+++ b/webrtc/modules/BUILD.gn
@@ -19,7 +19,7 @@ group("modules") {
     "audio_coding",
     "audio_conference_mixer",
     "audio_device",
-    "audio_mixer",
+    "audio_mixer:audio_mixer_impl",
     "audio_processing",
     "bitrate_controller",
     "desktop_capture",
@@ -628,6 +628,7 @@ if (rtc_include_tests) {
       "../system_wrappers:system_wrappers",
       "../test:rtp_test_utils",
       "../test:test_common",
+      "../test:test_support",
       "../test:test_support_main",
       "../test:video_test_common",
       "audio_coding",
@@ -644,7 +645,8 @@ if (rtc_include_tests) {
       "audio_coding:webrtc_opus",
       "audio_conference_mixer",
       "audio_device",
-      "audio_mixer",
+      "audio_mixer:audio_frame_manipulator",
+      "audio_mixer:audio_mixer_impl",
       "audio_processing",
       "audio_processing:audioproc_test_utils",
       "bitrate_controller",