GN: Replace mojo/system.gni with forwarding groups

media/mojo/services/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: media/media.gyp:mojo_media_lib
 source_set("lib") {
   deps = [
     "//base",
     "//media",
     "//media/mojo/interfaces"
     "//mojo/common",
     "//mojo/environment:chromium",
+    "//mojo/public/c/system:for_component",
   ]
 
-  deps += mojo_system_for_component
-
   sources = [
     "media_type_converters.cc",
     "media_type_converters.h",
   ]
 }
 
 test("mojo_media_lib_unittests") {
   sources = [
     "media_type_converters_unittest.cc",
   ]
 
   deps = [
     "//base/test:test_support",
     "//mojo/environment:chromium",
     "//testing/gtest",
     ":lib"
   ]
 }