This CL adds handling for cleaning up weakmap (EphemeronHashTable)
keys during scavenge, even if the weakmap resides in oldspace.

Change-Id: If8d711c050ddbcae4dd6e8da549e0c0d08ba47b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1523787
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60432}