GN: Replace mojo/system.gni with forwarding groups

mojo/aura/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("//mojo/system.gni")
-
 # GYP version: mojo/mojo.gyp:mojo_aura_support
 source_set("aura") {
   sources = [
     "aura_init.cc",
     "aura_init.h",
     "context_factory_mojo.cc",
     "context_factory_mojo.h",
     "screen_mojo.cc",
     "screen_mojo.h",
     "window_tree_host_mojo.cc",
     "window_tree_host_mojo.h",
     "window_tree_host_mojo_delegate.h",
   ]
 
   deps = [
     "//cc",
     "//skia",
     "//ui/aura",
     "//ui/compositor",
     "//ui/events",
     "//ui/events:events_base",
     "//ui/gl",
     "//webkit/common/gpu",
     "//mojo/cc",
+    "//mojo/public/gles2:for_shared_library",
+    "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/native_viewport",
-    "//mojo/services/public/cpp/view_manager",
-  ] + mojo_gles2_for_shared_library
+  ]
 
   forward_dependent_configs_from = [
     "//mojo/services/public/cpp/view_manager",
   ]
 }