Summary:
ArenaTest.MemoryAllocatedBytes on Travis failed:
https://travis-ci.org/facebook/rocksdb/jobs/79887849 . This is probably due to
malloc_usable_size() returning a value greater than the requested size. From
the man page:

   The value returned by malloc_usable_size() may be greater than the requested
   size of the allocation because of alignment and minimum size constraints.
   Although the excess bytes can be overwritten by the application without ill
   effects, this is not good programming practice: the number of excess bytes
   in an allocation depends on the underlying implementation.

Test Plan: make arena_test && ./arena_test

Reviewers: rven, anthony, yhchiang, aekmekji, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46743