GitLab

If we enforce that each new map value is greater than the previous one
then that ensures that no duplicates are allowed without doing a lookup
for every insert.

Bug: 827551
Change-Id: Idcc24b1070cc1707362f37a7f76198053882148a
Reviewed-on: https://chromium-review.googlesource.com/982610
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547546}