chromium/chrome/test/data/extensions/api_test/webstore_private/cancelled.html

<script src="common.js"></script>
<script>
// This tests the user cancelling an install started by
// beginInstallWithManifest3.

var manifest = getManifest();
// Begin installing.
chrome.webstorePrivate.beginInstallWithManifest3(
    {'id':extensionId, 'manifest': manifest},
    function(result) {
      assertEq(result, "user_cancelled");
      succeed();
    });
</script>