title: v8::Persistent MarkIndependent, what does this method exactly do?
body: <p>Following is v8 code comment, I don't quite understand explaination.</p>
  
  <pre class="lang-cpp prettyprint-override"><code>/**
   * Marks the reference to this object independent. Garbage collector
   * is free to ignore any object groups containing this object.
   * Weak callback for an independent handle should not
   * assume that it will be preceded by a global GC prologue callback
   * or followed by a global GC epilogue callback.
   */
  inline void MarkIndependent();
  inline void MarkIndependent(Isolate* isolate);
  </code></pre>