[Chromecast] Correct some Cast dependencies in //chrome/browser/ui.

ui/base/x/BUILD.gn

 # Copyright 2016 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/ui.gni")
+import("//ui/ozone/ozone.gni")
+
+assert(use_x11 || ozone_platform_x11)
 
 component("x") {
   output_name = "ui_base_x"
 
   sources = [
     "ui_base_x_export.h",
     "x11_menu_list.cc",
     "x11_menu_list.h",
     "x11_util.cc",
     "x11_util.h",
@@ skipping 10 matching lines @@
     "//base",
     "//base:i18n",
     "//skia",
     "//ui/events",
     "//ui/events/devices/x11",
     "//ui/events/keycodes:x11",
     "//ui/gfx",
     "//ui/gfx/x",
   ]
 }