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
diff --git a/src/gc/mark_sweep.cc b/src/gc/mark_sweep.cc
index f1f7267..d02e0cf 100644
--- a/src/gc/mark_sweep.cc
+++ b/src/gc/mark_sweep.cc
@@ -167,7 +167,7 @@
 }
 
 void MarkSweep::ProcessReferences(Thread* self) {
-  ReaderMutexLock mu(self, *Locks::heap_bitmap_lock_);
+  WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
   ProcessReferences(&soft_reference_list_, clear_soft_references_, &weak_reference_list_,
                     &finalizer_reference_list_, &phantom_reference_list_);
   timings_.AddSplit("ProcessReferences");