Move DisplayChangeObserver + tests from ash to ui.

ui/display/manager/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")
 
 component("manager") {
   sources = [
     "chromeos/apply_content_protection_task.cc",
     "chromeos/apply_content_protection_task.h",
     "chromeos/configure_displays_task.cc",
     "chromeos/configure_displays_task.h",
+    "chromeos/display_change_observer.cc",
+    "chromeos/display_change_observer.h",
     "chromeos/display_configurator.cc",
     "chromeos/display_configurator.h",
     "chromeos/display_layout_manager.h",
     "chromeos/display_snapshot_virtual.cc",
     "chromeos/display_snapshot_virtual.h",
     "chromeos/display_util.cc",
     "chromeos/display_util.h",
     "chromeos/query_content_protection_task.cc",
     "chromeos/query_content_protection_task.h",
     "chromeos/touchscreen_util.cc",
@@ skipping 46 matching lines @@
     configs += [
       "//build/config/linux:x11",
       "//build/config/linux:xext",
       "//build/config/linux:xi",
       "//build/config/linux:xrandr",
     ]
 
     deps += [ "//ui/events/platform" ]
   }
 }