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

ui/events/devices/x11/BUILD.gn

Index: ui/events/devices/x11/BUILD.gn
diff --git a/ui/events/devices/x11/BUILD.gn b/ui/events/devices/x11/BUILD.gn
index f6ab6d496c2cd752e767a8d5cba21f4e99eac9a6..a9c963c9b9a158e947fe7be443cbd4034e1eb32f 100644
--- a/ui/events/devices/x11/BUILD.gn
+++ b/ui/events/devices/x11/BUILD.gn
@@ -1,6 +1,10 @@
 # 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("x11") {
   output_name = "events_devices_x11"