[Chromecast] Replace architecure-based conditionals with flag.

ui/ozone/ozone.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")
 
 declare_args() {
   # Select platforms automatically. Turn this off for manual control.
   ozone_auto_platforms = true
 
@@ skipping 22 matching lines @@
     ozone_platform = "headless"
     ozone_platform_headless = true
 
     if (is_chromecast) {
       if (!disable_display) {
         # Enable the Cast ozone platform on all A/V Cast builds.
         ozone_platform_cast = true
 
         # For desktop Chromecast builds, override the default "headless"
         # platform with --ozone-platform=egltest
-        # TODO(slan|halliwell): Make the default platform "cast" on desktop too.
-        if (target_os == "linux" && target_cpu != "arm") {
+        # TODO(halliwell): Create a libcast_graphics implementation for desktop
+        # using X11, and disable these two platforms. "cast" platform should be
+        # the default on every A/V build.
+        if (is_cast_desktop_build) {
           ozone_platform_egltest = true
           ozone_platform_ozonex = true
         } else {
           # On device builds, enable "cast" as the default platform.
           ozone_platform = "cast"
         }
       }
     } else if (is_chromeos) {
       ozone_platform_gbm = true
       ozone_platform_egltest = true
     } else {
       # Build all platforms whose deps are in install-build-deps.sh.
       # Only these platforms will be compile tested by buildbots.
       ozone_platform_egltest = true
     }
   }
 }