Add is_chromecast to GN build.

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
 }
 
 declare_args() {
   # The platform that will be active by default.
   ozone_platform = ""
 
   # Enable individual platforms.
   ozone_platform_caca = false
+  ozone_platform_cast = false
   ozone_platform_egltest = false
   ozone_platform_gbm = false
   ozone_platform_ozonex = false
   ozone_platform_test = false
 
   if (ozone_auto_platforms) {
-    # Use test as the default platform.
-    ozone_platform = "test"
+    if (is_chromecast) {
+      # The default platform used at runtime is "cast".
+      ozone_platform = "cast"
 
-    # Build all platforms whose deps are in install-build-deps.sh.
-    # Only these platforms will be compile tested by buildbots.
-    ozone_platform_gbm = true
-    ozone_platform_test = true
-    ozone_platform_egltest = true
+      # For desktop Chromecast builds, override the default "cast" platform with
+      # --ozone_platform=egltest
+      ozone_platform_cast = true
+      ozone_platform_egltest = true
+      ozone_platform_ozonex = true
+      ozone_platform_test = true
+    } else {
+      # Use test as the default platform.
+      ozone_platform = "test"
+
+      # Build all platforms whose deps are in install-build-deps.sh.
+      # Only these platforms will be compile tested by buildbots.
+      ozone_platform_gbm = true
+      ozone_platform_test = true
+      ozone_platform_egltest = true
+    }
   }
 }