All present uses of the call-site arguments to MozPromise's methods
supply static strings. However, this is nowhere enforced. Do so.
Additionally, since this is the third or fourth time the present author
alone has personally implemented such an enforcement mechanism, create a
helper class to simplify doing so.
No functional changes.
Differential Revision: https://phabricator.services.mozilla.com/D207462
286effabaf