[Chromecast] Set use_alsa=true by default on Chromecast platforms.

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/features.gni")
 import("//build/config/headless_build.gni")
 
 declare_args() {
   # Allows distributions to link pulseaudio directly (DT_NEEDED) instead of
@@ skipping 45 matching lines @@
   is_openbsd = false
 }
 
 declare_args() {
   # 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 || is_cast_desktop_build)) {
+  # Alsa should be used on non-Android, non-Mac POSIX systems.
+  if (is_posix && !is_headless && !is_android && !is_mac) {
     use_alsa = true
-    if (!use_cras) {
+
+    # Pulse is not supported on Chromecast platforms.
+    if (!use_cras && !is_chromecast) {
       use_pulseaudio = true
     }
   }
 }