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 ec9de6248fcdd32c52fe590e11d77d424c635103..1909407a8c1a42b00dcd68f5dad4fd2e2be8677c 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1133,6 +1133,8 @@ 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