Passing in a Provider<CurrentUser> into PermissionBackend is
boiler-platy. This change adds a convenience method to PermissionBackend
and DefaultPermissionBackend to limit this boiler-plate. Future commits
will remove #user(Provider<CurrentUser>) from PermissionBackend, once
all callers were moved.
Change-Id: Ifcd07fe2c7d2673a66b2b4f9eff06ee8a3b6b943