GN: Replace mojo/system.gni with forwarding groups

mojo/examples/window_manager/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("//build/config/ui.gni")
-import("//mojo/system.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 assert(use_aura)
 
 # GYP version: mojo/mojo_examples.gypi:mojo_window_manager
 shared_library("window_manager") {
   output_name = "mojo_window_manager"
 
   sources = [
     "debug_panel.cc",
     "debug_panel.h",
     "window_manager.cc",
   ]
 
   deps = [
     ":bindings",
     "//base",
     "//mojo/application",
     "//mojo/aura",
     "//mojo/examples/keyboard:bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/cpp/input_events",
     "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/input_events",
     "//mojo/services/public/interfaces/navigation",
     "//mojo/services/window_manager:lib",
     "//mojo/views",
     "//ui/aura",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
     "//ui/resources",
     "//ui/views",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_window_manager_bindings
 mojom("bindings") {
   sources = [
     "window_manager.mojom",
   ]
   deps = [
     "//mojo/services/public/interfaces/geometry",
   ]
 }