title: chrome extension - force default_pop content
body: <p>current using content_scripts to produce an alert upon url match, example:</p>
  
  <pre><code>"content_scripts": [
      {
          "matches": ["https://www.google.co.uk/*"],
          "js": ["alert.js"]
      }
  ]
  </code></pre>
  
  <p>alert.js:</p>
  
  <pre><code>alert( 'Hello, world!' );
  </code></pre>
  
  <p>but rather than producing an alert is it possible to force open my default_open within manifest.json?</p>
  
  <pre><code>"default_popup": "index.html"
  </code></pre>