Support the serviceWorkerRegistration option for deleteToken

[HillBryan]

Discussion

• Only getToken accepts a serviceWorkerRegistration option.
• This MR adds the same option to deleteToken.
• Without this change, one must first call getToken at some point in the same session before deleteToken. Otherwise, the messaging instance will have no visibility to the custom service worker.

Testing

• Unit tests are included
• Manually tested

API Changes

• This is a backwards compatable API change to deleteToken.
• At this time we cannot accept changes that affect the public API. If you'd like to help
  us make Firebase APIs better, please propose your change in an issue so that we
  can discuss it together.