Convert certain DCHECKS into CHECKS for some Map setters. This should have
minimal performance impact at the same time getting us better coverage out
there in the wild.

Change-Id: I9a12f43e1baca15d9bf8b1aed86bb6b0dc13921d
Reviewed-on: https://chromium-review.googlesource.com/866931
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50958}