GN: Replace mojo/system.gni with forwarding groups

mojo/services/test_service/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")
 
 # GYP version: mojo/mojo_services.gypi:mojo_test_service_bindings
 mojom("bindings") {
   sources = [
     "test_request_tracker.mojom",
     "test_service.mojom",
   ]
 }
 
 # GYP version: mojo/mojo_services.gypi:mojo_test_app
 shared_library("mojo_test_app") {
   deps = [
     ":bindings",
     "//base",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [
     "test_request_tracker_client_impl.cc",
     "test_request_tracker_client_impl.h",
     "test_service_application.cc",
     "test_service_application.h",
     "test_service_impl.cc",
     "test_service_impl.h",
     "test_time_service_impl.cc",
     "test_time_service_impl.h",
   ]
 }
 
 # GYP version: //mojo/mojo_services.gypi:mojo_test_request_tracker_app
 shared_library("mojo_test_request_tracker_app") {
   deps = [
     ":bindings",
     "//base",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [
     "test_request_tracker_client_impl.cc",
     "test_request_tracker_client_impl.h",
     "test_request_tracker_application.cc",
     "test_request_tracker_application.h",
     "test_time_service_impl.cc",
     "test_time_service_impl.h",
     "test_request_tracker_impl.cc",
     "test_request_tracker_impl.h",
   ]
 }