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
--HG--
extra : rebase_source : 9c38c97e39e3c4fb192288d751505e1f0f2a2c6d
28c67cf8bd