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

Committed: https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b
Review-Url: https://codereview.chromium.org/2477073002
Cr-Original-Commit-Position: refs/heads/master@{#431020}
Cr-Commit-Position: refs/heads/master@{#431181}
7 files changed