WeakFixedArray::Shrink is fragile when not used properly (might invalidate GC
bookeeping (location of weak slots)).

BUG=v8:7308

Change-Id: Id84329e2a78907f5f0bfafae32fc2a71b77edbe7
Reviewed-on: https://chromium-review.googlesource.com/1076236Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53410}