Now that V8 uses EphemeronHashTable as backing store for
JSWeakCollections one pass over the heap is enough. In the old
implementation a second pass was necessary to find the owner
of the FixedArray to determine if its entries are strong or weak
references.

Bug: chromium:844008
Change-Id: I04bdf7d480c9be301831698571be5b226d20ac20
Reviewed-on: https://chromium-review.googlesource.com/1090910Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53643}