Fix incorrect lock for process references.
Fix incorrect lock for process references.

Process references occasionally calls ProcessMarkStack which requires
exclusive access to the heap bitmap lock. Fixed an error where we had
a reader lock here instead of a writer lock.

Change-Id: I4740f7e80fe2c73ff822a770f250c33698ae0dd1

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst