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

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 5918237b13ee536438c4faffc55a7287ce5899ad..449825e4e09a81161c840e057dc03c5d6a290ed3 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -63,12 +63,12 @@ declare_args() {
   # 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
     }
   }