In support for Website isolation, we need to tweak Frame::mainFrame() so that it returns an AbstractFrame object instead of a Frame, and then callers will have to deal with both LocalFrame and RemoteFrame.
rdar://105412703
Committed 260414@main (52ec37994c2d): <https://commits.webkit.org/260414@main> Reviewed commits have been landed. Closing PR #9958 and removing active labels.