GN: Replace mojo/system.gni with forwarding groups

mojo/examples/dbus_echo/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")
-
 group("dbus_echo") {
   deps = [
     ":dbus_echo_app",
     ":dbus_echo_service",
   ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_dbus_echo
 shared_library("dbus_echo_app") {
   output_name = "mojo_dbus_echo"
 
   sources = [
     "dbus_echo_app.cc",
   ]
 
   deps = [
     "//base",
     "//mojo/application",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/examples/echo:bindings",
-  ] + mojo_system_for_shared_library
+  ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_dbus_echo_service
 executable("dbus_echo_service") {
   output_name = "mojo_dbus_service"
 
   sources = [
     "dbus_echo_service.cc",
   ]
 
   deps = [
     "//base",
     "//dbus",
     "//mojo/application",
     "//mojo/common",
     "//mojo/dbus",
     "//mojo/examples/echo:bindings",
     "//mojo/system",
   ]
 }