Move DisplayChangeObserver + tests from ash to ui.

ui/base/user_activity/user_activity_detector.cc

Index: ui/base/user_activity/user_activity_detector.cc
diff --git a/ui/base/user_activity/user_activity_detector.cc b/ui/base/user_activity/user_activity_detector.cc
index bfda1cfdc91a96587501abf2cd021daaa48b3b24..69c3b998d49ff8e7394f9e7a372db69db630f2b9 100644
--- a/ui/base/user_activity/user_activity_detector.cc
+++ b/ui/base/user_activity/user_activity_detector.cc
@@ -71,6 +71,12 @@ UserActivityDetector* UserActivityDetector::Get() {
   return g_instance;
 }
 
+// static
+void UserActivityDetector::OnDisplayPowerChangingStatic() {
+  if (g_instance)
+    g_instance->OnDisplayPowerChanging();
+}
+
 bool UserActivityDetector::HasObserver(
     const UserActivityObserver* observer) const {
   return observers_.HasObserver(observer);