chromeos: Convert system tray session length limit to mojo

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 121f24539ac1f98607b70ae99f87f5819fe4429b..d5d81b978eb137ece5bb84fcd7961461347f3b3f 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -237,7 +237,8 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
   // Otherwise it could be a main axis margin on the tray's box layout.
   AddTrayItem(base::MakeUnique<PaddingTrayItem>());
 
-  AddTrayItem(base::MakeUnique<TraySessionLengthLimit>(this));
+  tray_session_length_limit_ = new TraySessionLengthLimit(this);
+  AddTrayItem(base::WrapUnique(tray_session_length_limit_));
   tray_enterprise_ = new TrayEnterprise(this);
   AddTrayItem(base::WrapUnique(tray_enterprise_));
   tray_supervised_user_ = new TraySupervisedUser(this);
@@ -600,6 +601,11 @@ TrayNetwork* SystemTray::GetTrayNetworkForTesting() const {
   return tray_network_;
 }
 
+TraySessionLengthLimit* SystemTray::GetTraySessionLengthLimitForTesting()
+    const {
+  return tray_session_length_limit_;
+}
+
 TraySupervisedUser* SystemTray::GetTraySupervisedUserForTesting() const {
   return tray_supervised_user_;
 }