[Chromecast] Replace architecure-based conditionals with flag.

media/media_options.gni

 # Copyright 2014 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/headless_build.gni")
 
 declare_args() {
   # Allows distributions to link pulseaudio directly (DT_NEEDED) instead of
   # using dlopen. This helps with automated detection of ABI mismatches and
@@ skipping 20 matching lines @@
 
   # Enables runtime selection of PulseAudio library.
   use_pulseaudio = false
 
   # Enables runtime selection of ALSA library for audio.
   use_alsa = false
 
   # Alsa should be used on non-Android, non-Mac POSIX systems, and Chromecast
   # builds for desktop Linux.
   if (is_posix && !is_headless && !is_android && !is_mac &&
-      (!is_chromecast || target_cpu != "arm")) {
+      (!is_chromecast || is_cast_desktop_build)) {
     use_alsa = true
     if (!use_cras) {
       use_pulseaudio = true
     }
   }
 
   # Use low-memory buffers on non-Android builds of Chromecast.
   use_low_memory_buffer = is_chromecast && !is_android
 
   # Enables the MPEG2-TS stream parser for use with Media Source. Disabled by
   # default (except on Chromecast) since it's not available on the normal Web
   # Platform and costs money.
   enable_mpeg2ts_stream_parser = is_chromecast
 
   # Enable HEVC/H265 demuxing. Actual decoding must be provided by the
   # platform. Enable by default for Chromecast.
   enable_hevc_demuxing = is_chromecast
 
   # Experiment to enable mojo media application: http://crbug.com/431776
   # Valid options are:
   # - "none": Do not use mojo media application.
   # - "browser": Use mojo media application hosted in the browser process.
   # - "gpu": Use mojo media application hosted in the gpu process.
   # - "utility": Use mojo media application hosted in the utility process.
   enable_mojo_media = "none"
 
   # TODO(GYP): This should be a platform define.
   is_openbsd = false
 }