servo: Merge #17728 - Reuse bloom filter and style sharing cache across traversals (from emilio:reuse-allocs); r=bholley,emilio

This relands #17701, with a fix for the test failures.

Source-Repo: https://github.com/servo/servo
Source-Revision: f49650ce96578c200c71b0a8fa50094764a3c76d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : c766e6a5e7cb14d45b722f73fb6b63b1f50d97e2