Add countdown when session time is limited

ash/system/tray/system_tray_notifier.cc

Index: ash/system/tray/system_tray_notifier.cc
diff --git a/ash/system/tray/system_tray_notifier.cc b/ash/system/tray/system_tray_notifier.cc
index e865550fb63a675ef5bb84bf3f0b50891290bc99..77af895a472083c3489a11dfba985cf908138550 100644
--- a/ash/system/tray/system_tray_notifier.cc
+++ b/ash/system/tray/system_tray_notifier.cc
@@ -107,6 +107,16 @@ void SystemTrayNotifier::RemovePowerStatusObserver(
   power_status_observers_.RemoveObserver(observer);
 }
 
+void SystemTrayNotifier::AddSessionLengthLimitObserver(
+    SessionLengthLimitObserver* observer) {
+  session_length_limit_observers_.AddObserver(observer);
+}
+
+void SystemTrayNotifier::RemoveSessionLengthLimitObserver(
+    SessionLengthLimitObserver* observer) {
+  session_length_limit_observers_.RemoveObserver(observer);
+}
+
 void SystemTrayNotifier::AddUpdateObserver(UpdateObserver* observer) {
   update_observers_.AddObserver(observer);
 }
@@ -243,6 +253,20 @@ void SystemTrayNotifier::NotifyPowerStatusChanged(
                     OnPowerStatusChanged(power_status));
 }
 
+void SystemTrayNotifier::NotifySessionStartTimeChanged(
+    const base::Time& session_start_time) {
+  FOR_EACH_OBSERVER(SessionLengthLimitObserver,
+                    session_length_limit_observers_,
+                    OnSessionStartTimeChanged(session_start_time));
+}
+
+void SystemTrayNotifier::NotifySessionLengthLimitChanged(
+    const base::TimeDelta& limit) {
+  FOR_EACH_OBSERVER(SessionLengthLimitObserver,
+                    session_length_limit_observers_,
+                    OnSessionLengthLimitChanged(limit));
+}
+
 void SystemTrayNotifier::NotifyUpdateRecommended(
     UpdateObserver::UpdateSeverity severity) {
   FOR_EACH_OBSERVER(UpdateObserver,