Use Gecko's simpler Bloom filter instead of one based on hash stretching.

This preserves the usage of the Bloom filter throughout style recalc,
but the implementation is rewritten. Provides a 15% improvement on
Guardians of the Galaxy.

r? @zwarich
f? @cgaebel 
If you don't box, then taking and putting the bloom filter in TLS actually copies the whole thing. Before this patch, we were copying the entire bloom filter twice per node, which made memmove dominate the profile.