[Chromecast] Implement new volume control API

chromecast/chromecast.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chromecast_build.gni")
 
 # This args block should contain arguments used within the //chromecast
 # directory. Arguments which are used in other Chrome components should
 # be instead declared in //build/config/chromecast_build.gni.
 declare_args() {
   # chromecast_branding is used to include or exclude Google-branded components.
   # Set it to "public" for a Chromium build.
   chromecast_branding = "public"
 
   # The incremental build number. The Cast automated builders will set this
   # value to indicate the buildset. Note: The default value should be greater
   # than any value the builder may assign to prevent attempted automatic updates
   # when the default value is used.
   cast_build_incremental = "999999"
 
   # If true, IS_CAST_DEBUG_BUILD() will evaluate to 1 in version.h. Otherwise,
   # it will evaluate to 0. Overriding this when is_debug=false is useful for
   # doing engineering builds.
   cast_is_debug = is_debug
 
   # If true, Chromecast WebUI resources are included in a build.
   # 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() {
   # Use Playready CDMs for internal non-desktop builds.
   use_playready = !is_cast_desktop_build && chromecast_branding != "public"
 }
 
 # This is the release version, which takes the form <major>.<minor>. Internal
 # builds will read the value from an internal file containing this value as a
 # string. Public builds, lacking this file, will set a dummy value.
 if (chromecast_branding == "public") {
   cast_build_release = "eng." + getenv("USER")
 } else {
   cast_build_release =
       read_file("//chromecast/internal/build/cast_build_release", "trim string")
 }