Write animated values into the ComputedValues structures when

animations complete or are interrupted.

This adds a new pair of reader-writer locks. I measured the performance
of style recalculation on Wikipedia and the overhead of the locks was
not measurable.

Closes #7816.

components/layout/lib.rs

@@ -5,6 +5,7 @@
 #![feature(box_syntax)]
 #![feature(cell_extras)]
 #![feature(custom_derive)]
+#![feature(drain)]
 #![feature(hashmap_hasher)]
 #![feature(mpsc_select)]
 #![feature(plugin)]