Fix the placement of arrows in the zoom box

Now that [bug 649849](https://bugzilla.mozilla.org/show_bug.cgi?id=649849) has been fixed, adding support for `-moz-appearance: none`, the arrow is now too close to the text in the zoom box. This is currently only an issue in Nightly, but assuming the patch doesn't get backed out, this will soon affect all versions of Firefox.

This is an illustration of the issue:
![zoom-arrow](https://cloud.githubusercontent.com/assets/2692120/4515904/e8880e90-4bd7-11e4-9395-b44d3cb73401.png)

I tried using a CSS only solution, but didn't find one that worked well both with and without bug 649849. (Of course removing `-moz-appearance: none` from our CSS would "work", but that doesn't seem like real solution.)