GN: Replace mojo/system.gni with forwarding groups

mojo/services/native_viewport/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("//mojo/system.gni")
 
 # GYP version: mojo/mojo_services.gypi:mojo_native_viewport_service_lib
 static_library("native_viewport") {
   output_name = "mojo_native_viewport"
 
   deps = [
     "//base",
     "//cc/surfaces",
     "//mojo/application",
     "//mojo/common",
@@ skipping 45 matching lines @@
     deps += [
       "//ui/events/platform/x11",
       "//ui/platform_window/x11",
     ]
   }
 
   if (use_ozone) {
     sources += [ "platform_viewport_ozone.cc" ]
   }
 }