Most are brought over straightforwardly, their Telemetry callsites reworded
to use Glean, with mirroring to the Telemetry probes taken care of by the Glean
Interface For Firefox Telemetry (see the telemetry_mirror property).
There were two special cases: PERF_DOM_CONTENT_LOADED_FROM_RESPONSESTART_MS,
and PERF_DOM_CONTENT_LOADED_MS. Both were removed as they were unused.
Differential Revision: https://phabricator.services.mozilla.com/D200867
