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It was using the SpawnedTestServer, which is slow to start, and has
been a never ending source of flake.

HeadlessBrowserTest.ServerWantsClientCertificate will continue to use
the SpawnedTestServer, since the EmbeddedTestServer does not support
client cert requests.

Bug: 492672
Change-Id: I03a511abf63ee149704c908e838275f07b4e9903
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508652
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822727}