Bug 41562: do not force letterboxing on API-initiated fullscreen.

This fixes #41562 (closed) by reverting to the previous non-letterboxing behavior on DOM-side fullscreen initiated by the requestFullscreen() API.

If we decide we want this case to be letterboxed as well, that can be worked out as a different patch (after looking at the intricacies, probably not a cakewalk), but in the meanwhile we've got this to remove the page-cropping regression and make the new direct-sizing letterboxing machinery a better replacement for the old one.