GN: Replace mojo/system.gni with forwarding groups

mojo/examples/media_viewer/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/public/tools/bindings/mojom.gni")
-import("//mojo/system.gni")
 
 if (use_aura) {
 
 # GYP version: mojo/mojo_examples.gypi:mojo_media_viewer
 shared_library("media_viewer") {
   output_name = "mojo_media_viewer"
 
   sources = [
     "media_viewer.cc"
   ]
 
   deps = [
     ":bindings",
     "//base",
     "//mojo/application",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/cpp/input_events",
     "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/navigation",
     "//mojo/services/public/interfaces/view_manager",
     "//mojo/views",
     "//skia",
     "//ui/gfx/geometry",
     "//ui/views",
-  ] + mojo_system_for_shared_library
+  ]
 }
 
 }  # use_aura
 
 # GYP version: mojo/mojo_examples.gypi:mojo_media_viewer_bindings
 mojom("bindings") {
   sources = [
     "media_viewer.mojom",
   ]
 }