[Chromecast] Rename disable_display flag to is_cast_audio_only.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 09e1cc2280e782389522e25842d89b666d109a95..ac3a5da3b8ae46a5be67306ad9d88e85974c4bea 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -57,7 +57,7 @@ declare_args() {
   # Alsa should be used on non-Android, non-Mac POSIX systems.
   # Alsa should be used on desktop Chromecast and audio-only Chromecast builds.
   if (is_posix && !is_android && !is_mac &&
-      (!is_chromecast || is_cast_desktop_build || disable_display)) {
+      (!is_chromecast || is_cast_desktop_build || cast_audio_only)) {
     use_alsa = true
 
     # Pulse is not supported on Chromecast platforms.