Don't record post-quantum stats if MITM.

If something other than Google is terminating the TLS connection, then
we shouldn't expect the post-quantum ciphersuites to be present.  So,
exclude such connections from the statistics.

(Suggestion from estark--thanks!)

BUG=626363

Review-Url: https://codereview.chromium.org/2251483002
Cr-Commit-Position: refs/heads/master@{#412100}
1 file changed