Within iframes, content script should defer action to the top-level window

[jchang504]

A quick, manifest-only change enabled the content script to be injected into iframes, including dynamically generated ones (fixing #25). But it's not a complete solution, because now the UI runs within the iframe, which looks particularly weird with small windows like on hangouts.google.com. The only case where it's a more than cosmetic problem is tab search, where the results can be obscured within a small window. I think it's still rather an edge case, and the work-to-UX-gains ratio is high.

To fix it, we can follow an approach like in https://stackoverflow.com/a/12904047, which is where I started for the solution to #25.