Move DisplayChangeObserver + tests from ash to ui.

ui/display/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")
 import("//testing/test.gni")
 
 component("display") {
   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 156 matching lines @@
     "//ui/gfx:test_support",
     "//ui/gfx/geometry",
   ]
 }
 
 # This test covers all testable components in display.
 test("display_unittests") {
   sources = [
     "chromeos/apply_content_protection_task_unittest.cc",
     "chromeos/configure_displays_task_unittest.cc",
+    "chromeos/display_change_observer_unittest.cc",
     "chromeos/display_configurator_unittest.cc",
     "chromeos/query_content_protection_task_unittest.cc",
     "chromeos/touchscreen_util_unittest.cc",
     "chromeos/update_display_configuration_task_unittest.cc",
     "display_change_notifier_unittest.cc",
     "display_list_unittest.cc",
     "display_unittest.cc",
     "fake_display_snapshot_unittests.cc",
     "manager/display_layout_builder_unittest.cc",
     "manager/display_layout_unittest.cc",
@@ skipping 28 matching lines @@
     sources -= [ "display_change_notifier_unittest.cc" ]
   }
 
   if (is_chromeos && use_x11) {
     sources += [
       "chromeos/x11/display_util_x11_unittest.cc",
       "chromeos/x11/native_display_event_dispatcher_x11_unittest.cc",
     ]
   }
 }