The GC now stores EphemeronHashTables in a Worklist.

Bug: chromium:844008
Change-Id: I6ad95b3b49484901d75c6fd92b59065c438fcd03
Reviewed-on: https://chromium-review.googlesource.com/1093092
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53620}