- Get rid of fixing up pointers on the main thread
- Get rid of sweeping on the main thread

Instead:
- Record (and process afterwards) slots in parallel
- Add the pages to the concurrent sweeper as pointers have already been fixed

BUG=chromium:581412
LOG=N
TEST=cctest/test-compaction/*

Review URL: https://codereview.chromium.org/1881423003

Cr-Commit-Position: refs/heads/master@{#35451}