GN: Replace mojo/system.gni with forwarding groups

mojo/examples/aura_demo/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")
 
 assert(use_aura)
 
 # GYP version mojo/mojo_examples.gypi:mojo_aura_demo
 shared_library("mojo_aura_demo") {
   output_name = "mojo_aura_demo"
 
   sources = [
     "aura_demo.cc"
   ]
 
   deps = [
     "//base",
     "//cc",
     "//ui/aura",
     "//ui/base",
     "//ui/compositor",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//mojo/application",
     "//mojo/common",
     "//mojo/aura",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/cpp/view_manager",
-  ] + mojo_system_for_shared_library
+  ]
 
   datadeps = [ ":init" ]
 }
 
 # GYP version mojo/mojo_examples.gypi:mojo_aura_demo_init
 shared_library("init") {
   output_name = "mojo_aura_demo_init"
 
   sources = [
     "view_manager_init.cc",
   ]
 
   deps = [
     "//base",
     "//mojo/application",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/view_manager",
-  ] + mojo_system_for_shared_library
+  ]
 }