added a missing hook to IndexedMemPool's testing harness

Summary: IndexedMemPool had one bare usage of std::atomic, rather than
the templated type Atom.  This doesn't affect any non-testing template
instantiations, because those two are usually synonyms, but it could cause
spurious failures of DeterministicSchedule tests.  Found via inspection,
not via failed tests.

Reviewed By: @yfeldblum

Differential Revision: D2277424

folly/IndexedMemPool.h

@@ -306,7 +306,7 @@ struct IndexedMemPool : boost::noncopyable {
   /// To allow use of atomic ++ instead of CAS, we let this overflow.
   /// The actual number of constructed elements is min(actualCapacity_,
   /// size_)
-  std::atomic<uint32_t> size_;
+  Atom<uint32_t> size_;
 
   /// raw storage, only 1..min(size_,actualCapacity_) (inclusive) are
   /// actually constructed.  Note that slots_[0] is not constructed or used