This is a reland of 4f945599

Original change's description:
> Add Visitor for EphemeronHashTable
> 
> JSWeakCollection does not have any weak references anymore. Special
> handling of Ephemerons can now be implemented in
> VisitEphemeronHashTable.
> 
> Bug: chromium:844008
> Change-Id: I9f4d8ad6a32cc7a55b715803f6a83ff8d2743ce8
> Reviewed-on: https://chromium-review.googlesource.com/1090274
> Commit-Queue: Dominik Inführ <dinfuehr@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53584}

Bug: chromium:844008
Change-Id: I9d2953d55eb0affba4fede5ec37369c1eb6574f5
Reviewed-on: https://chromium-review.googlesource.com/1092370Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53602}