JSWeakCollection should use EphemeronHashTable as backing store instead of
ObjectHashTable such that the GC can handle these structures differently in
the future.

Bug: chromium:844008
Change-Id: Icc6df60c975a942877e2507ef45e0d235e5f72be
Reviewed-on: https://chromium-review.googlesource.com/1089063
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53572}