When call getAttribute, HTMLIFrame.sandbox has a bug which don't return changed value.

Source/core/html/HTMLIFrameElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Simon Hausmann (hausmann@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Ericsson AB. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 151 matching lines @@
 {
     return true;
 }
 
 void HTMLIFrameElement::valueChanged()
 {
     String invalidTokens;
     setSandboxFlags(m_sandbox->value().isNull() ? SandboxNone : parseSandboxPolicy(m_sandbox->tokens(), invalidTokens));
     if (!invalidTokens.isNull())
         document().addConsoleMessage(ConsoleMessage::create(OtherMessageSource, ErrorMessageLevel, "Error while parsing the 'sandbox' attribute: " + invalidTokens));
+    setSynchronizedLazyAttribute(sandboxAttr, m_sandbox->value());
 }
 
 }