[Chromecast] Implement new volume control API

chromecast/chromecast.gni

Index: chromecast/chromecast.gni
diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index 6f717052c180504eca4fa7faca70e0e78f66fb91..ec300c53d1e5e531c6b7e639d146946d9866be7a 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni
@@ -27,6 +27,9 @@ declare_args() {
   # TODO(antz): default to false for audio-only builds, might need further
   # clean up (b/27119303)
   enable_chromecast_webui = !is_cast_audio_only && !is_android
+
+  # Set true to enable assistant features.
+  enable_assistant = false

[slan, 2017/03/13 22:09:51]

The assert from internal should be copied at the end of this file:

# Assistant shall only be enabled on audio-only builds.
assert(!enable_assistant || is_cast_audio_only,
       "Assistant is only available on audio devices.")

[kmackay, 2017/03/14 00:20:59]

Done.

 }
 
 declare_args() {