GN: Replace mojo/system.gni with forwarding groups

mojo/examples/apptest/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/public/tools/bindings/mojom.gni")
-import("//mojo/system.gni")
 
 group("apptest") {
   testonly = true
 
   deps = [
     ":apptests",
     ":service",
   ]
 }
 
 # GYP version mojo/mojo_examples.gypi:mojo_example_service
 shared_library("service") {
   output_name = "mojo_example_service"
 
   sources = [
     "example_service_application.cc",
     "example_service_application.h",
     "example_service_impl.cc",
     "example_service_impl.h",
   ]
 
   deps = [
     ":bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/utility"
-  ] + mojo_system_for_shared_library
+  ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_example_apptests
 shared_library("apptests") {
   output_name = "mojo_example_apptests"
 
   testonly = true
 
   sources = [
     "example_apptest.cc",
     "example_client_application.cc",
     "example_client_application.h",
     "example_client_impl.cc",
     "example_client_impl.h",
   ]
 
    deps = [
     ":bindings",
     "//testing/gtest",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/utility",
-   ] + mojo_system_for_shared_library
+   ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_example_service_bindings
 mojom("bindings") {
   sources = [ "example_service.mojom" ]
 }