title: Modify the response body of HTTP requests with Chrome extension
body: <p>I am working on a Chrome extension. I would like to modify some contents when the (very first) page load happens.
  I wonder if I use the method <code>getContent()</code> in <code>chrome.devtools.network.onRequestFinished.addListener</code>, how can I modify the content after it is returned from the method?</p>
  
  <p>Is this method only for network/traffic observation? Can I modify the response body? </p>