Add new atomic operations and use them in two places.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 27933e3e7621363344752d132e0f08e3cc8f8652..be297390aee4d4e533216ade7a0be15361b4a7ce 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -646,6 +646,11 @@ class Object {
     raw()->StorePointer(addr, value);
   }
 
+  template<typename type>
+  void AtomicStorePointer(type const* addr, type value) const {
+    raw()->AtomicStorePointer(addr, value);
+  }
+
   // Store a range of pointers [from, from + count) into [to, to + count).
   // TODO(koda): Use this to fix Object::Clone's broken store buffer logic.
   void StorePointers(RawObject* const* to,