If the user's cleanup function didn't iterate all available WeakCells, we need
to schedule the cleanup task again at some point. The previous condition
resulted it never being scheduled.

BUG=v8:8179

Change-Id: I8f5f4c01d1eb6a3cca8bd21bdc52c38663889882
Reviewed-on: https://chromium-review.googlesource.com/c/1286686
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56772}