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chromium
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chromium/chrome/test/data/extensions/background_app/BackgroundApp.html 

<!doctype html>
<html>
  <head>
    <title>Background App Popup</title>
    <!-- JavaScript and HTML must be in separate files for security. -->
    <script src="BackgroundApp.js"></script>
  </head>
  <body>
    Simple Background App
  </body>
</html>