Add is_chromecast to GN build.

build/config/ui.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.
 
 # This file contains UI-related build flags (see features.gni for Chrome
 # feature-related ones). These should theoretically be moved to the ui
 # directory.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
 # e.g. base, so they need to be global to match the GYP configuration. Also,
 # anything that needs a grit define must be in either this file or features.gni.
 #
 # PLEASE TRY TO AVOID ADDING FLAGS TO THIS FILE in cases where grit isn't
 # required. See the declare_args block of BUILDCONFIG.gn for advice on how
 # to set up feature flags.
 
+import("//build/config/chromecast_build.gni")
+
 declare_args() {
   # Indicates if Ash is enabled. Ash is the Aura Shell which provides a
   # desktop-like environment for Aura. Requires use_aura = true
-  use_ash = is_win || is_linux
+  use_ash = (is_win || is_linux) && !is_chromecast
 
   # Indicates if Ozone is enabled. Ozone is a low-level library layer for Linux
-  # that does not require X11.
-  use_ozone = false
+  # that does not require X11. Enabling this feature disables use of glib, x11,
+  # Pango, and Cairo. Default to false on non-Chromecast builds.
+  use_ozone = is_chromecast
 
   # Indicates if Aura is enabled. Aura is a low-level windowing library, sort
   # of a replacement for GDI or GTK.
-  use_aura = is_win || is_linux
+  use_aura = is_win || is_linux || is_chromecast
 
   # True means the UI is built using the "views" framework.
-  toolkit_views = is_mac || is_win || is_chromeos || use_aura
+  toolkit_views =
+      (is_mac || is_win || is_chromeos || use_aura) && !is_chromecast
 
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
 
   # Whether we should use GTKv3 instead of GTKv2.
   use_gtk3 = false
 }
 
 # Additional dependent variables -----------------------------------------------
 #
@@ skipping 20 matching lines @@
 }
 
 # Whether to use atk, the Accessibility ToolKit library
 use_atk = is_desktop_linux && use_x11
 
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
 enable_hidpi = is_mac || is_win || is_linux
 
 enable_topchrome_md = is_chromeos || is_win || is_linux