Add is_chromecast to GN build.

chromecast/graphics/BUILD.gn

 # Copyright 2015 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("//chromecast/chromecast.gni")
+
 source_set("graphics") {
   sources = [
     "cast_screen.cc",
     "cast_screen.h",
   ]
 
   deps = [
     "//ui/aura",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 }
+
+group("libcast_graphics") {
+  if (use_default_cast_graphics) {
+    public_deps = [
+      ":libcast_graphics_default",
+    ]
+  } else {
+    public_deps = [
+      "$libcast_graphics_path:libcast_graphics_1.0",
+    ]
+  }
+}
+
+if (use_default_cast_graphics) {
+  shared_library("libcast_graphics_default") {
+    output_name = "libcast_graphics_1.0"
+
+    sources = [
+      "cast_egl_platform_default.cc",
+      "graphics_properties_default.cc",
+      "osd_plane_default.cc",
+    ]
+
+    public_deps = [
+      "//chromecast/public",
+    ]
+
+    deps = [
+      "//base",
+    ]
+  }
+}