Fix GN toolchains to use proper out dir.

mojo/services/surfaces/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_services.gypi:mojo_surfaces_service
 shared_library("surfaces") {
   output_name = "mojo_surfaces_service"
 
   deps = [
     "//base",
     "//cc",
     "//cc/surfaces",
     "//ui/gfx/geometry",
     "//mojo/environment:chromium",
     "//mojo/public/cpp/application:chromium",
     "//mojo/cc",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/surfaces",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/cpp/surfaces",
   ] + mojo_gles2_for_shared_library + mojo_system_for_shared_library
 
   sources = [
     "surfaces_impl.cc",
     "surfaces_impl.h",
     "surfaces_service_application.cc",
     "surfaces_service_application.h",
+    "surfaces_service_impl.cc",
+    "surfaces_service_impl.h",
   ]
 }