chromeos: Convert system tray session length limit to mojo

ash/system/tray/system_tray_delegate.cc

Index: ash/system/tray/system_tray_delegate.cc
diff --git a/ash/system/tray/system_tray_delegate.cc b/ash/system/tray/system_tray_delegate.cc
index a825be27060f5e5b0bc2b265312fa80997a69ca1..82c4ced6b7740042533d7a1f6da3e14fb92ee795 100644
--- a/ash/system/tray/system_tray_delegate.cc
+++ b/ash/system/tray/system_tray_delegate.cc
@@ -31,16 +31,6 @@ NetworkingConfigDelegate* SystemTrayDelegate::GetNetworkingConfigDelegate()
   return nullptr;
 }
 
-bool SystemTrayDelegate::GetSessionStartTime(
-    base::TimeTicks* session_start_time) {
-  return false;
-}
-
-bool SystemTrayDelegate::GetSessionLengthLimit(
-    base::TimeDelta* session_length_limit) {
-  return false;
-}
-
 void SystemTrayDelegate::ActiveUserWasChanged() {}
 
 bool SystemTrayDelegate::IsSearchKeyMappedToCapsLock() {