This is an intermediate step for concurrent slot filtering. This CL already makes filtering concurrent, but does not integrate it in the actual sweeping. This will be done in two follow up CLs. One for the regular slot set and one for the typed slot set.

BUG=chromium:648568

Review-Url: https://codereview.chromium.org/2401563002
Cr-Commit-Position: refs/heads/master@{#40040}