All information about malloc zones [and their original functions] is stored in
MallocZoneAggregator. When the allocator shim is ready to dispatch back to the
original system implementations, it calls the MallocZoneAggregator with
|context|, which provides enough information to dispatch to the correct malloc
zone.

BUG=693237

Review-Url: https://codereview.chromium.org/2703803004
Cr-Commit-Position: refs/heads/master@{#452784}