Bug 1712928 - Gather telemetry about encoding-unlabeled pages and about Repair Text Encoding usage situations. r=emk

In particular, gather telemetry to evaluate the impact of unlabeled UTF-8
and how detector-triggered reloads would change if ASCII-only at initial
guess was treated as UTF-8.

Differential Revision: https://phabricator.services.mozilla.com/D140818