Make mutable version of AllBrowsingContextsIterator

[vimpunk]

Currently we need to iterate over all browsing contexts in a frame tree and save their IDs in a Vec. Then, we need to iterate over them and retrieve the actual BrowsingContext instances, again and do the actual procedure of setting the is_visible field and calling Pipeline::notify_visibility. Here's the code.

It'd be nice not to have to make that copy in the beginning and lookup each browsing context twice, and instead mutate the browsing contexts in place as we iterate over them with a mutable version of AllBrowsingContextsIterator, but I'm not sure if this is possible (alas, my rust-lifetime-fu is not very advanced yet).