With the removal of the old addonHistograms, all histograms are now registered.
So removing registered(Keyed)Histograms should be straightforward?
Unfortunately not, as this was how we filtered data based on dataset
(opt-in/opt-out), so a little more fiddling was needed to get C++ to only
serialize dataset-appropriate data (instead of post-facto filtering it in JS).
MozReview-Commit-ID: HDplhmzmzJl
Some tests were using "secret" APIs to get at telemetry knowledge from various
processes in various ways. Adjust them so that they work with the new ways of
doing things.
MozReview-Commit-ID: 2iBL00HVGyg
