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 
Why does this just take the lower 32 bits, when `Atom::bloom_hash` xors with the top 32 bits?