[Chromecast] Replace architecure-based conditionals with flag.

chromecast/chromecast.gni

Index: chromecast/chromecast.gni
diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index fa36ec9810e8b3a335180e1df36cc24baa5962ad..7c2c7761b9259140353895bed5c41aced7ffc7c3 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni
@@ -12,9 +12,6 @@ declare_args() {
   # Set it to "public" for a Chromium build.
   chromecast_branding = "public"
 
-  # True if Chromecast build is targetted for linux desktop.
-  is_chromecast_desktop_build = is_linux && target_cpu != "arm"
-
   # Use Playready CDMs.
   use_playready = false
 }