Move hash code from hidden string to a private symbol

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 2a37a3aa324dbf0c2dbf995d18eb748588f03d29..bdc3210ca2b0424c405862a45e06f64923db4196 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1134,6 +1134,8 @@ class Isolate {
  private:
   friend struct GlobalState;
   friend struct InitializeGlobalState;
+  Handle<JSObject> SetUpSubregistry(Handle<JSObject> registry, Handle<Map> map,
+                                    const char* name);
 
   // These fields are accessed through the API, offsets must be kept in sync
   // with v8::internal::Internals (in include/v8.h) constants. This is also