Move Slot recording to concurrent and incremental marking phase and
out of the atomic pause. Records more slots to remove
slot recording completely from the pause.

Bug: chromium:844008
Change-Id: I13ba20b4839990134dc37bd909fb0a2d66e13c64
Reviewed-on: https://chromium-review.googlesource.com/1104348
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53804}