The next CL will add an actual WeakFixedArray which contains in-place weak
references.

Also removes FLAG_trace_weak_arrays which is not super useful.

BUG=v8:7308

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I016880ecc66b03b406f7184b7f72ab514cb65428
Reviewed-on: https://chromium-review.googlesource.com/951730
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51789}