[Chromecast] Enable MultizoneBackendTest on Earth.

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")
 import("//build/config/locales.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.
@@ skipping 18 matching lines @@
     display_web_contents_in_service = is_cast_audio_only
   }
 
   # 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
+
+  # Set to true on devices that support multizone. Only indicates that the
+  # device supports multizone. The command line flag --enable-multizone must
+  # still be used to enable multizone.
+  supports_multizone = is_cast_audio_only && !is_cast_desktop_build
 }
 
 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")
 }
 
 # All locales supported by Cast builds. This provides a single point of
 # reference for all GN files referencing a locales list. |locales| is declared
 # in //build/config/locales.gni.
 cast_locales = locales