Fix style editor to work on chrome windows

[harthur]

This PR will allow the Style Editor to debug chrome window targets. I'm using this scratchpad to test it: https://gist.github.com/4a1c22f41e99da2511ae

[harthur]

My bad, works on Dev Tools windows but not browser windows. But, it's progress, still think it could be checked in.

[harthur]

jk, it did work on browser windows, it just so happened most browser css files had an @import rule at the beginning and that was causing a HierarchyRequestError (can't insert before an @import rule). This last commit fixes that by inserting the transition rules at the end of the stylesheet.

t+

[paulrouget]

I'm surprised how simple this is: http://i.imgur.com/zvpEu.jpg

[paulrouget]

Heather, does your bugzilla patch include your latest commit?

[paulrouget]

We landed that.