Add a headless configuration to Mojo's native viewport service

mojo/services/native_viewport/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")
 
 if (!is_android) {
   shared_library("native_viewport") {
     output_name = "mojo_native_viewport_service"
 
@@ skipping 37 matching lines @@
 
   sources = [
     "gpu_impl.cc",
     "gpu_impl.h",
     "native_viewport_impl.cc",
     "native_viewport_impl.h",
     "platform_viewport.h",
     "platform_viewport_android.cc",
     "platform_viewport_android.h",
     "platform_viewport_mac.mm",
+    "platform_viewport_headless.cc",
+    "platform_viewport_headless.h",
     "platform_viewport_win.cc",
     "viewport_surface.cc",
     "viewport_surface.h",
   ]
 
   if (is_ios) {
     sources += [ "platform_viewport_stub.cc" ]
   }
 
   if (is_android) {
     deps += [ "//mojo:jni_headers" ]
   }
 
   if (use_x11) {
     sources += [ "platform_viewport_x11.cc" ]
     deps += [
       "//ui/events/platform/x11",
       "//ui/platform_window/x11",
     ]
   }
 
   if (use_ozone) {
     sources += [ "platform_viewport_ozone.cc" ]
   }
 }