When using WebExt messaging with Responsive Design Mode, message manager
messages traveling from content to parent such as "MessageChannel:Messages" are
received with `event.target` set to the <iframe mozbrowser> inside RDM that
hosts the content.
Here we change `getSender` processing for messages like this to also check HTML
iframes for related tabs, in addition to XUL elements as before.
MozReview-Commit-ID: 3qphe2W8jHM