Move some code from Runtime_SetPrototype to JSObject::SetPrototype.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 37618b42fc6aa2a64f1c7d627e94f6ae3a19593f..73eaf19e151e7f1bb9e6d85d4f92aeda699b5f66 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2521,6 +2521,9 @@ class JSObject: public JSReceiver {
   MUST_USE_RESULT static Maybe<bool> PreventExtensionsWithTransition(
       Handle<JSObject> object, ShouldThrow should_throw);
 
+  MUST_USE_RESULT static MaybeHandle<Object> SetPrototypeUnobserved(
+      Handle<JSObject> object, Handle<Object> value, bool from_javascript);
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSObject);
 };