It's a growing weak array which also has an API for marking slots empty (those
will then be filled before growing the array again).

This is a more efficient implementation than the corresponding feature in
FixedArrayOfWeakCells, because we chain the empty slots together.

BUG=v8:7308

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I335cd3f9cc7838c7f6ca350735b1503b2f5b8eed
Reviewed-on: https://chromium-review.googlesource.com/1090922
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54481}