title: run javascript in overrided new tab page
body: <p>i've written a little chrome extension in order to replace the new tab page. everythings is fine, but some javascript won't run on my local html file (which replaces ntp).
  i'm totally new to chrome extension. i think i miss something in the manifest. can someone please help me?
  here's my manifest</p>
  
  <pre><code>{
      "chrome_url_overrides": 
      {
          "newtab": "inicio.html"    
      },
      "description": "Mostra una pagina vuota quando si apre una nuova scheda",    
      "incognito":"split",    
      "manifest_version": 2,    
      "name": "Nuova Scheda Vuota",   
      "version": "1.0" 
  }
  </code></pre>
  
  <p>index.html  uses some xxx.js file in js folder beloging to ext folder</p>