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chromium/chrome/test/data/extensions/api_test/extension_resource_request_policy/web_accessible/iframe.html 

<!DOCTYPE html>
<html>
<body>
  <script src="iframe.js"></script>

  <!-- Iframe resource from the same extension, this should load -->
  <iframe src="iframe-contents.html"></iframe>

  <!-- Iframe resource from another extension that's not web accessible, this should not load -->
  <iframe src="chrome-extension://pbkkcbgdkliohhfaeefcijaghglkahja/inaccessible-iframe-contents.html"></iframe>
</body>
</html>