Add is_chromecast to GN build.

ui/ozone/ozone.gni

Index: ui/ozone/ozone.gni
diff --git a/ui/ozone/ozone.gni b/ui/ozone/ozone.gni
index df32eb7392924ec49a8c5b076e529e8b7ba796f1..e6be8c4128c69a81e850320ba9954777bf92e88b 100644
--- a/ui/ozone/ozone.gni
+++ b/ui/ozone/ozone.gni
@@ -2,9 +2,11 @@
 # 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
+  ozone_auto_platforms = !is_chromecast
 }
 
 declare_args() {
@@ -13,6 +15,7 @@ declare_args() {
 
   # Enable individual platforms.
   ozone_platform_caca = false
+  ozone_platform_cast = false
   ozone_platform_egltest = false
   ozone_platform_gbm = false
   ozone_platform_ozonex = false
@@ -26,5 +29,15 @@ declare_args() {
     # Only these platforms will be compile tested by buildbots.
     ozone_platform_test = true
     ozone_platform_egltest = true
+  } else if (is_chromecast) {
+    # The default platform used at runtime is "cast".
+    ozone_platform = "cast"
+
+    # 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
   }
 }