This modifies the ObjectVisitor to provide a dedicated VisitEphemeron
method invoked when visiting a EphemeronHashTable. This is pre-work
for further changes to how ephemerons are handled during scavenging.

Bug: v8:8557
Change-Id: Ia423b10667ec222cbe5f44d8a931ea33314625f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1508673Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60119}