chromeos: Convert system tray session length limit to mojo

ash/session/session_controller.cc

Index: ash/session/session_controller.cc
diff --git a/ash/session/session_controller.cc b/ash/session/session_controller.cc
index 20d884a368939adabc0c910d8ea82529881dd2f6..6ebfff7a36818dfb164777dedb9da8c6393a22d0 100644
--- a/ash/session/session_controller.cc
+++ b/ash/session/session_controller.cc
@@ -280,6 +280,14 @@ void SessionController::NotifyChromeTerminating() {
     observer.OnChromeTerminating();
 }
 
+void SessionController::SetSessionLengthLimit(base::TimeDelta length_limit,
+                                              base::TimeTicks start_time) {
+  session_length_limit_ = length_limit;
+  session_start_time_ = start_time;
+  for (auto& observer : observers_)
+    observer.OnSessionLengthLimitChanged();
+}
+
 void SessionController::ClearUserSessionsForTest() {
   user_sessions_.clear();
 }