Bug 1102138 - [gaia-header] Should no longer reverse content in RTL mode

I always feel bad when running some code when the script is merely executed. I also wonder if it would be better to create an in-memory element and call querySelector on it, instead of doing so on document which can have a bigger tree. My main issue is that _when_ `:host` is supported, then the querySelector call will likely walk the whole DOM tree.

What do you think?