[proxies] Make Object.prototype.isPrototypeOf work with proxies.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 24c8c14f1ced7a1b8a266342cf287cf81f22b18a..a7b5a68da0e0527f68ed08b8945418cc2087a6e1 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1197,16 +1197,10 @@ MaybeHandle<Object> Object::GetPrototype(Isolate* isolate,
       !isolate->MayAccess(context, Handle<JSObject>::cast(receiver))) {
     return isolate->factory()->null_value();
   }
-  if (receiver->IsJSProxy()) {
-    return JSProxy::GetPrototype(Handle<JSProxy>::cast(receiver));
-  }
   PrototypeIterator iter(isolate, receiver,
                          PrototypeIterator::START_AT_RECEIVER);
   do {
-    iter.AdvanceIgnoringProxies();
-    if (PrototypeIterator::GetCurrent(iter)->IsJSProxy()) {
-      return PrototypeIterator::GetCurrent(iter);
-    }
+    if (!iter.AdvanceFollowingProxies()) return MaybeHandle<Object>();
   } while (!iter.IsAtEnd(PrototypeIterator::END_AT_NON_HIDDEN));
   return PrototypeIterator::GetCurrent(iter);
 }