GN: Replace mojo/system.gni with forwarding groups

mojo/services/network/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_services.gypi:mojo_network_service
 shared_library("network") {
   output_name = "mojo_network_service"
 
   deps = [
     ":lib",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/network",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "main.cc" ]
 }
 
 # GYP version: mojo/mojo_services.gypi:mojo_network_service_lib
 source_set("lib") {
   deps = [
     "//base",
     "//mojo/application",
     "//mojo/common",
     "//mojo/environment:chromium",
     "//mojo/services/public/interfaces/network",
     "//net",
     "//url",
   ]
 
   sources = [
     "cookie_store_impl.cc",
     "cookie_store_impl.h",
     "network_context.cc",
     "network_context.h",
     "network_service_impl.cc",
     "network_service_impl.h",
     "url_loader_impl.cc",
     "url_loader_impl.h",
     "web_socket_impl.cc",
     "web_socket_impl.h",
   ]
 }