Add locking to the heap.

Now tsan is happy with us too, at least on the host.

Change-Id: Ib4657f56be2014de832dff8886b63843a40ea788
diff --git a/src/mark_sweep.cc b/src/mark_sweep.cc
index 307a804..c947da3 100644
--- a/src/mark_sweep.cc
+++ b/src/mark_sweep.cc
@@ -151,7 +151,7 @@
   Space* space = static_cast<Space*>(arg);
   for (size_t i = 0; i < num_ptrs; ++i) {
     Object* obj = static_cast<Object*>(ptrs[i]);
-    Heap::RecordFree(space, obj);
+    Heap::RecordFreeLocked(space, obj);
     space->Free(obj);
   }
   // TODO, unlock heap if concurrent