title: Communicate with <webview> in Electron
body: <p>I have a <code>&lt;webview&gt;</code> in my Electron app. I'd like to have safe "foreign" communication, the way I would with an <code>iframe</code> through <code>postMessage</code>. So for example:</p>
  
  <p><code>webview.executeJavaScript("window.parent.postMessage('all done!')");
  </code></p>
  
  <p>Is my only choice for communication with this subwebview to turn on <code>nodeIntegration</code> so that I can use <code>sendToHost</code>? Turning on all of <code>nodeIntegration</code> just for this one feature seems like overkill.</p>