Use correct LifetimeActor method to trigger deletion of object.

Ran into this when fixing an issue in lifetime manager.

Clients should call Delete instead of simply enqueueing the DeleteActor
to the LifetimeManager.  The main difference is that the Delete method
has protection against duplicate Enqueue.

Will separately make changes to LifetimeManager code to make Enqueue a
private method.

Change-Id: Ic6224d822e37d07aa067244c77490e17965e5db1
Merge "Use correct LifetimeActor method to trigger deletion of object."