Keep initial values in a lazy static instead of passing a parameter around.

Thanks to @Kimundi for https://gist.github.com/Kimundi/8782487
Add a fast path in the cascade for anonymous boxes.
auto merge of #2416 : SimonSapin/servo/static-initial-values, r=larsberg

Thanks to @Kimundi for https://gist.github.com/Kimundi/8782487

r? @larsbergstrom