GN: Replace mojo/system.gni with forwarding groups

mojo/examples/sample_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_sample_app
 shared_library("sample_app") {
   output_name = "mojo_sample_app"
 
   sources = [
     "gles2_client_impl.cc",
     "gles2_client_impl.h",
     "sample_app.cc",
   ]
 
   deps = [
     ":spinning_cube",
     "//gpu/command_buffer/client:gles2_interface",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/native_viewport",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }
 
 source_set("spinning_cube") {
   sources = [
     "spinning_cube.cc",
     "spinning_cube.h"
   ]
 
   deps = [
     "//base",
-  ] + mojo_gles2_for_shared_library
+    "//mojo/public/gles2:for_shared_library",
+  ]
 }