title: window onload event not fired in chrome content script
body: <p>I use content script in my Chrome extension. content scripts are injected at "document_start".<br> </p>
  
  <pre><code>window.onload = function() {console.log("window onload event fired.");}
  </code></pre>
  
  <p>I run the above code in content script, but when I load a page, the onload event is not fired.
  <br>Is there anything wrong?</p>