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This is an initial solution for crbug.com/1129197 -- the full solution
is implemented in crrev.com/c/2422435 but requires changes to allow
mocking CT results in order to land.

This prevents logging private certificates, and adds a test explicitly
exercising this case. It also makes the browser tests a bit more robust
by tracking the last report seen by the test server and adding a step to
flush a new report through for negative tests.

Bug: 1129197
Change-Id: I7e1d4010b2666db7f98194aa1e3ba80df1e0a493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453777Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815086}