[Chromecast] Replace architecure-based conditionals with flag.

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"
 
-  # True if Chromecast build is targetted for linux desktop.
-  is_chromecast_desktop_build = is_linux && target_cpu != "arm"
-
   # Use Playready CDMs.
   use_playready = false
 }