TB 43254: Allow some DomainFrontedRequests to be cancelled.

This is used by some of the Moat calls to stop abandoned requests
running in the background:

1. Cancelled AutoBootstraps.
2. Cancelled CAPTCHA request/submissions.

fixup! TB 31286: Implementation of bridge, proxy, and firewall settings in about:preferences#connection

TB 43254: Cancel Moat requests when the CAPTHCA dialog is closed.