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This allows path building to continue and try to build a valid path
(either to a different root, or to the same cert if it happens to be
self-signed.)

Update comments & todos that trusted leaf certs are intentionally not
supported.

Bug: 814994
Change-Id: Id8e6a5f3d00c94c96271e4c6e21860206bb71c2a
Reviewed-on: https://chromium-review.googlesource.com/933108Reviewed-by: Doug Steedman <dougsteed@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538880}