The extension doesn’t work with elements in iframes

This is caused by a bug on Chromium’s end which I can’t work around: https://crbug.com/40281393

The bug being that when selecting an element inside an iframe, the reported value for `$0` returns `undefined` when calling [`chrome.devtools.inspectedWindow.eval`](https://developer.chrome.com/docs/extensions/reference/devtools_inspectedWindow/#method-eval).

Because this bug needs to be fixed on Chrome’s end, I am marking this issue as https://github.com/bramus/containing-block-devtools-extension/labels/external and https://github.com/bramus/containing-block-devtools-extension/labels/wontfix (more like _“cantfix”_, right?). To make sure this issue can be found, I’m keeping this one open and am also adding it to the https://github.com/bramus/containing-block-devtools-extension/labels/FAQ.