Mask out xhprof for memory allocation tracking

Summary:
When xhprof is enabled, it will allocate quite a bit of memory
and this can throw off normal memory allocation statistics.  Mask these
allocations since it is not relevant for measurements.

Test Plan:
xhprof_enable() followed by memory_get_allocation() seeing no
increase in allocation memory.

Reviewers: je

Reviewed By: je

CC: jasont, ps, mwilliams, je

Differential Revision: 349035

src/runtime/base/memory/memory_manager.h

@@ -191,6 +191,7 @@ class MemoryManager {
 #ifdef USE_JEMALLOC
       // exclude mallocs and frees since construction
       if (s_statsEnabled) {
+        m_mm->m_prevAllocated = int64(*m_mm->m_allocated);
         m_mm->m_delta = int64(*m_mm->m_allocated) - int64(*m_mm->m_deallocated);
       }
 #endif