GN: Replace mojo/system.gni with forwarding groups

mojo/services/window_manager/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")
 
 if (use_aura) {
 
-import("//mojo/system.gni")
-
 # GYP version: mojo/mojo_services.gypi:mojo_core_window_manager
 shared_library("window_manager") {
   output_name = "mojo_core_window_manager"
 
   sources = [ "main.cc" ]
 
   deps = [
     ":lib",
     "//mojo/application",
-  ] + mojo_system_for_shared_library
+    "//mojo/public/c/system:for_shared_library",
+  ]
 }
 
 # GYP version: mojo/mojo_services.gypi:mojo_core_window_manager_lib
 source_set("lib") {
   sources = [
     "window_manager_app.cc",
     "window_manager_app.h",
     "window_manager_service_impl.cc",
     "window_manager_service_impl.h",
   ]
@@ skipping 40 matching lines @@
       "//third_party/mesa:osmesa",
       "//mojo/services/native_viewport",
     ]
   }
   if (use_x11) {
     deps += [ "//ui/gfx/x" ]
   }
 }
 
 }  # use_aura