The previous implementation of this used the iframe’s `src` uri as the key for identifying which frames needed to be removed from the sidebar’s frame-sync state.
We can’t rely on this uri because there could be one or more iframes sharing the same src uri, for example ‘about:blank’ with/without `srcdoc` attribute usage.
Another problem is with a `blob:` uri as the src. Once again the uri in the src can’t be used as the key since it won’t match what frame-sync thinks the proper uri is if it’s being substituted with  `baseURI`.

The way this works now is by tagging the frame info with a random ID called the frameIdentifier. The handling of this tag is coordinated with the relevant modules, cross frame, bridge, frame-sync, etc.