Once the broker retired, only allow retrieval of a reference's
handle. That's all we need for now.

Bug: v8:7790
Change-Id: Ib75887ed4a68e19ad7fad4c6046e340502542850
Reviewed-on: https://chromium-review.googlesource.com/1251086Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56308}