Add a headless configuration to Mojo's native viewport service

mojo/mojo_services.gypi

 # Copyright (c) 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.
 
 {
   'targets': [
     {
       # GN version: //mojo/services/clipboard/
       'target_name': 'mojo_clipboard',
       'type': 'loadable_module',
@@ skipping 346 matching lines @@
         'mojo_native_viewport_bindings',
         'mojo_surfaces_bindings',
       ],
       'sources': [
         'services/native_viewport/gpu_impl.cc',
         'services/native_viewport/gpu_impl.h',
         'services/native_viewport/native_viewport_impl.cc',
         'services/native_viewport/native_viewport_impl.h',
         'services/native_viewport/platform_viewport.h',
         'services/native_viewport/platform_viewport_android.cc',
+        'services/native_viewport/platform_viewport_headless.cc',
+        'services/native_viewport/platform_viewport_headless.h',
         'services/native_viewport/platform_viewport_mac.mm',
         'services/native_viewport/platform_viewport_ozone.cc',
         'services/native_viewport/platform_viewport_stub.cc',
         'services/native_viewport/platform_viewport_win.cc',
         'services/native_viewport/platform_viewport_x11.cc',
         'services/native_viewport/viewport_surface.cc',
         'services/native_viewport/viewport_surface.h',
       ],
       'conditions': [
         ['OS=="win" or OS=="android" or OS=="linux" or OS=="mac"', {
@@ skipping 598 matching lines @@
               'dependencies': [
                 '../ui/gfx/x/gfx_x11.gyp:gfx_x11',
               ],
             }],
           ],
         },
       ],
     }],
   ],
 }