GN: Add assert that //components/exo is referenced only on supported platforms

components/exo/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("//build/config/ui.gni")
 import("//testing/test.gni")
 
+assert(enable_wayland_server)
+
 source_set("exo") {
   sources = [
     "buffer.cc",
     "buffer.h",
     "display.cc",
     "display.h",
     "keyboard.cc",
     "keyboard.h",
     "keyboard_delegate.h",
     "pointer.cc",
@@ skipping 99 matching lines @@
   deps = [
     ":unit_tests",
     "//ash:test_support",
     "//base",
     "//base/test:test_support",
   ]
   if (is_linux) {
     deps += [ "//components/exo/wayland:unit_tests" ]
   }
 }