[Chromecast] Replace architecure-based conditionals with flag.

build/config/chromecast_build.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.
 
 # This block should only contain Chromecast-specific build arguments used by
 # by components outside of //chromecast.
 declare_args() {
   # Set this true for a Chromecast build. Chromecast builds are supported on
   # Linux and Android.
   is_chromecast = false
 
+  # True if Chromecast build is targeted for linux desktop. This type of build
+  # is useful for testing and development, but currently supports only a subset
+  # of Cast functionality. Though this defaults to true for x86 Linux devices,
+  # this should be overriden manually for an embedded x86 build.
+  # TODO(slan): Remove instances of this when x86 is a fully supported platform.
+  is_cast_desktop_build =
+      is_chromecast && is_linux && (target_cpu == "x86" || target_cpu == "x64")
+
   # Set this true for an audio-only Chromecast build.
   disable_display = false
 }
 
 # Assert that Chromecast is being built for a supported platform.
 assert(is_linux || is_android || !is_chromecast,
        "Chromecast builds are not supported on $target_os")
 
-# Assert that disable_display is not true on a non-Chromecast build.
-assert(!disable_display || is_chromecast)
+# Assert that disable_display and is_cast_desktop_build are both false on a
+# non-Chromecast build.
+assert(is_chromecast || !(disable_display || is_cast_desktop_build))