A follow up will allow inserting slots during GC without emiting fences

Bug: chromium:651354
Change-Id: Ia1d0f88e3658bca31933bdb013db15a5c2ecd849
Reviewed-on: https://chromium-review.googlesource.com/541400
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46042}