Adding UMA to track previews opt outs and blacklist eligibility

This CL adds UMA to track the reason that a user was not shown a preview
(or if it was allowed) and whether a user opts out when shown a preview.

BUG=647717

Review-Url: https://codereview.chromium.org/2477073002
Cr-Commit-Position: refs/heads/master@{#431020}
7 files changed