title: QWebEnginePage: toHtml returns an empty string
body: <p>I need to retrieve some html from a <code>QWebEnginePage</code>. I found in the documentation the method <a href="http://doc.qt.io/qt-5/qwebenginepage.html#toHtml" rel="nofollow">toHtml</a> but it always returns an empty string. I tried <a href="http://doc.qt.io/qt-5/qwebenginepage.html#toPlainText" rel="nofollow">toPlainText</a>
   and it works, but this is not what I need.</p>
  
  <pre><code>MyClass::MyClass(QObject *parent) : QObject(parent)
  {
     _wp = new QWebEnginePage();
     _wp-&gt;settings()-&gt;setAttribute(QWebEngineSettings::AutoLoadImages, false);
     _wp-&gt;settings()-&gt;setAttribute(QWebEngineSettings::JavascriptEnabled, true);
     connect(_wp, SIGNAL(loadFinished(bool)), this, SLOT(wpLoadFinished(bool)));
  }
  void MyClass::start()
  {
     _wp-&gt;load(QUrl("http://google.com/"));
  }
  void MyClass::wpLoadFinished(bool s)
  {
     _wp-&gt;toHtml(
         [] (const QString &amp;result) {
            qDebug()&lt;&lt;"html:";
            qDebug()&lt;&lt;result;
      }); // return empty string
      /*_wp-&gt;toPlainText(
         [] (const QString &amp;result) {
            qDebug()&lt;&lt;"txt:";
            qDebug()&lt;&lt;result;
      });*/ //works perfectly
  }
  </code></pre>
  
  <p>What am I doing wrong?</p>