GN: Replace mojo/system.gni with forwarding groups

mojo/examples/compositor_app/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_examples.gypi:mojo_compositor_app
 shared_library("compositor_app") {
   output_name = "mojo_compositor_app"
 
   sources = [
     "compositor_app.cc",
     "compositor_host.cc",
     "compositor_host.h",
   ]
 
   deps = [
     "//base",
     "//cc",
     "//mojo/application",
     "//mojo/cc",
     "//mojo/common",
+    "//mojo/public/c/system:for_shared_library",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/gpu",
     "//mojo/services/public/interfaces/native_viewport",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }