Fix the MessageLoop type in case more than one ozone platform is built

content/gpu/BUILD.gn

 # 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.
 
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//ui/ozone/ozone.gni")
 
 # See //content/BUILD.gn for how this works.
 group("gpu") {
@@ skipping 83 matching lines @@
   if (is_chromeos && current_cpu != "arm") {
     configs += [ "//third_party/libva:libva_config" ]
   }
 
   if (use_x11) {
     deps += [ "//ui/events/platform/x11" ]
   }
 
   if (use_ozone) {
     deps += [ "//ui/ozone" ]
-
-    if (ozone_platform_x11) {
-      defines = [ "OZONE_X11" ]
-    }
   }
 
   if (enable_vulkan) {
     deps += [ "//gpu/vulkan" ]
   }
 }