We have been using a different zone allocator between mozjemalloc and
replace-malloc for a long time. Jemalloc 4 uses the same as
replace-malloc, albeit as part of the jemalloc upstream code base.

We've been bitten many times in the past with Apple changes breaking the
zone allocator, and each time we've had to make changes to the three
instances, although two of them are similar and the changes there are
straightforward.

It also turns out that the way the mozjemalloc zone allocator is set up,
when a new version of OSX appears with a new version of the system zone
allocator, Firefox ends up using the system allocator, because the zone
allocator version is not supported.

So, we use the same zone allocator for both replace-malloc and
mozjemalloc, making everything on par with jemalloc 4.

--HG--
extra : rebase_source : 9c0e245b5f82bb71294370d607e690c05cc89fbc