Clients may use mementos to persist interesting state between sessions.
Clients should ensure that the memento keys are unique; generally by using
the id of the content extension as a prefix.
Restore the previous state of any actions using the flags in aMemento.
This method allows the state of any actions that persist from session to
session to be restored.
Parameters: aMemento - A memento that was given to the view part to restore itsstate.