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This also allows us to support asynchronous client certificate
verification, but this CL leaves it as a TODO for now.

Bug: 706445
Change-Id: I792eb91a854bb15a67317d7ea4d04a80ba5ca4da
Reviewed-on: https://chromium-review.googlesource.com/586431Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491886}