Since LinearHistogram and its descendants inherit ranges_ from
Histogram, and we wanted to replace the copying into a std::vec
for Histogram, the simplest approach seemed to just be to
precompute ranges for all histograms, exponential or otherwise.
This should have the added benefit of reducing the memory
footprint for those histograms, since they will benefit from the
deduplication work that the precomputing script already does.

MozReview-Commit-ID: JTV5Dej5ZIb

--HG--
extra : rebase_source : de942d54b3475be54c70d43d2fa8e772ee2e18c4