GitLab

This matches Chromium's behaviour, part of which (unfortunately) is
defined in the //content layer in the form of DCHECKs.

BUG=

Change-Id: I4ea29bec34eb3348aa2640db55b1cef88443490f
Reviewed-on: https://chromium-review.googlesource.com/747222Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512866}