Add PlatformScreenDelegate and start implementation.

services/ui/display/BUILD.gn

 # Copyright 2016 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")
 
 source_set("display") {
   sources = [
     "platform_screen.h",
+    "platform_screen_delegate.h",
   ]
 
   deps = [
     "//base",
     "//ui/display",
     "//ui/gfx",
   ]
 
   if (use_ozone && is_chromeos) {
     sources += [
       "platform_screen_impl_ozone.cc",
       "platform_screen_impl_ozone.h",
     ]
 
     deps += [
       "//skia",
       "//ui/ozone:ozone",
     ]
   } else {
     sources += [
       "platform_screen_impl.cc",
       "platform_screen_impl.h",
     ]
   }
 }