Currently we clear only old-to-new slots in the sweeper.
For old-to-old slots we maintain the invariant that there are no
recorded slots in dead objects by explicitly clearing them
on object size change and array trimming.

The write barrier for concurrent marking will record slots
even when the host object is white. Thus, it can introduce slots
in dead objects, which will break evacuation phase if we do not
clear them in the sweeper.

Besides that, the patch makes handling of slots more uniform and
allows us to remove clearing of slots on object size changes.

BUG=chromium:694255

Change-Id: I48f60eb25ddc48c6948be4461367e3f7abf74672
Reviewed-on: https://chromium-review.googlesource.com/592207Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46996}