The link between the JS weak collection object and its backing store
was missing.

Change-Id: If8293a8d43fb52bc4fc9f156ccda578233a1991c
Reviewed-on: https://chromium-review.googlesource.com/579267Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46787}