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This adds a previews type state to the pingback that represents that no
preview was served due to the blacklist acting on one of its rules. The
states are still mutually exclusive.

Bug: 849328
Change-Id: I40235d318b68e9674b67ae40df30153c30aba178
Reviewed-on: https://chromium-review.googlesource.com/1086242
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564744}