auto merge of #1598 : pcwalton/servo/sequential-mirrors-parallel, r=larsbergstrom

in the parallel path.

r? @larsbergstrom
auto merge of #1600 : pcwalton/servo/use-concurrent-hash-map, r=larsbergstrom

64% improvement in style recalc.

r? @larsbergstrom
layout: Construct flows in parallel