JSWeakCollections used to be chained in a singly-linked list. The GC now
stores the JSWeakCollection's backing store EphemeronHashTable in a
Worklist. This should allow easier parallelization in the future.

Bug: chromium:844008
Change-Id: Icc0c1b537c23e8d24dc5b27452fb8106562c9a09
Reviewed-on: https://chromium-review.googlesource.com/1092496
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53619}