Skip over tiny frames when calling focusThisFrame

This fixes (hopefully?) #1317 by implementing the idea in [this comment](https://github.com/philc/vimium/issues/1317#issuecomment-67046634).
We're adding lots of great small usability improvements throughout the code, but in so doing, we should make sure to give future readers enough context to understand why it's there. In this case, mentioning Google's Inbox is helpful, as well as the issue number. It looks like @smblott-github added a comment later.