Follows-up e4ac1ef. A lot of this class gets its own cache object
in key-generation methods because of legacy compat with public
methods and because of how wfMemcKey used to be.
However where possible, we should encourage passing $cache
so that makeKey() can be used on the same instance that uses
the key.
This is a no-op since it's exactly the same object.
Change-Id: Ib3c31110176659a9175679eb716369e7f0a1d3b2
