Add mojom::DisplayController.

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")
 import("//testing/test.gni")
 
 source_set("display") {
   sources = [
+    "platform_screen.cc",
     "platform_screen.h",
     "platform_screen_delegate.h",
   ]
 
   deps = [
     "//base",
     "//ui/display",
     "//ui/gfx",
   ]
 
@@ skipping 28 matching lines @@
       "//base/test:test_support",
       "//testing/gmock",
       "//testing/gtest",
       "//ui/display",
       "//ui/gfx",
       "//ui/gfx:test_support",
       "//ui/ozone",
     ]
   }
 }