This CL fixes uses of HandleSlackTracking which previously would write
fields twice. Additional checks ensure that only proper initial maps are
used with slack tracking.

Change-Id: Ifb03297635ed6b873eb8b27fec9794c9d36b71b6
Reviewed-on: https://chromium-review.googlesource.com/664810Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49331}