Also add comments and regression tests.

Bug: chromium:904272
Change-Id: I89e8ec537bbdce09fda120cd29d5a5e54e77cf19
Reviewed-on: https://chromium-review.googlesource.com/c/1335556Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57500}