Also, workaround issues with QUnit Chrome bridge: the Chrome bridge
from `grunt-contrib-qunit` is now getting injected into every single
iframe, including an empty one that has no intention of running QUnit
tests. Since that bridge requires QUnit, it fails with an error
in such cases. Workaround the issue by wrapping the bridge in
another function that bails early if QUnit is not defined.
Ref gh-2157
