Read value of custom attributes of tabs via TSTBase.getTabAttribute() safely.

Tab's ID and other attributes saved via setTabValue is not available before the tab is completely restored.
As the result, TST sometimes fail to restore tab positions. (See: #676)
This issue doesn't happen on Firefox 24ESR, because recent Firefox (29 and later) uses message manager to restore tabs and it causes such "delay" of initializations about custom attributes.