The work list should be empty in the case we are not tearing down
during incremental marking. Explicitly clear during incremental marking
and rely on DCHECKs for the other case.

Bug: chromium:758570
Change-Id: Iecc8935f0cbfda67f0e7afde42930050e7c94d2d
Reviewed-on: https://chromium-review.googlesource.com/674504Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48087}