Put zapping of local handles behind the extra checks flag.

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index 3e408b73f234e06353f6a9186d339b68258d8c84..bcd611307b8e23e682a042868e1f243f5c4211ce 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -160,13 +160,14 @@ class HandleScope {
   // Extend the handle scope making room for more handles.
   static internal::Object** Extend(Isolate* isolate);
 
+#ifdef ENABLE_EXTRA_CHECKS
   // Zaps the handles in the half-open interval [start, end).
   static void ZapRange(internal::Object** start, internal::Object** end);
+#endif
 
-  friend class v8::internal::DeferredHandles;
   friend class v8::HandleScope;
+  friend class v8::internal::DeferredHandles;
   friend class v8::internal::HandleScopeImplementer;
-  friend class v8::ImplementationUtilities;
   friend class v8::internal::Isolate;
 };