Previously we only allowed the viewport itself to be the containing
block for fixed-position elements, but the specs give us a few other
situations. Many of these are the same as for absolute-positioned ones.
We were failing to actually climb up the containing block chain,
causing this API to infinite loop for anything but the most
trivial cases.
By fixing the loop structure, we also make a bunch of the already
imported WPT tests pass. :^)
