GN: Replace mojo/system.gni with forwarding groups

mojo/examples/nesting_app/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_examples.gypi:mojo_nesting_app
 shared_library("nesting_app") {
   output_name = "mojo_nesting_app"
 
   sources = [
     "nesting_app.cc"
   ]
 
   deps = [
     "//base",
     "//ui/gfx/geometry",
     "//ui/gl",
     "//url",
     "//mojo/application",
     "//mojo/examples/window_manager:bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/navigation",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }