Support 64-bit volatiles and reference/pointer volatiles.

Change-Id: I62c0751747767c02d6c57afd0fce3db6c8b02510
diff --git a/src/heap.h b/src/heap.h
index 52c1f10..773789b 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -137,6 +137,10 @@
   // Callers must hold the heap lock.
   static void RecordFreeLocked(Space* space, const Object* object);
 
+  // Must be called if a field of an Object in the heap changes, and before any GC safe-point.
+  // The call is not needed if NULL is stored in the field.
+  static void WriteBarrier(const Object* object);
+
  private:
   // Allocates uninitialized storage.
   static Object* AllocateLocked(size_t num_bytes);