GN: Replace mojo/system.gni with forwarding groups

mojo/examples/png_viewer/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_png_viewer
 shared_library("png_viewer") {
   output_name = "mojo_png_viewer"
 
   sources = [
     "png_viewer.cc"
   ]
 
   deps = [
     "//mojo/application",
     "//mojo/examples/media_viewer:bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/content_handler",
     "//mojo/services/public/interfaces/network",
     "//skia",
     "//ui/gfx",
-  ] + mojo_system_for_shared_library
+  ]
 }