I'm not sure what this issue is about. Please explain in more detail. Thanks!

The PreviewsBlackList should track how often the blacklist is not loaded from the DB yet when it is asked whether a preview should be shown. More generally, the reason a preview is not shown should be tracked in UMA.

Also, the opt out vs non-opt out navigations should be tracked in UMA.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc61592651c3018a146387f691bc4cbd60f7807b

commit cc61592651c3018a146387f691bc4cbd60f7807b
Author: ryansturm <ryansturm@chromium.org>
Date: Wed Nov 09 20:17:11 2016

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}

[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/BUILD.gn
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/previews_black_list.cc
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/previews_black_list.h
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/previews_black_list_unittest.cc
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/previews_io_data.cc
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/components/previews/core/previews_io_data_unittest.cc
[modify] https://crrev.com/cc61592651c3018a146387f691bc4cbd60f7807b/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/032b6e1db3c05363d2db3d2323573ff3fad3512a

commit 032b6e1db3c05363d2db3d2323573ff3fad3512a
Author: dpranke <dpranke@chromium.org>
Date: Thu Nov 10 00:16:22 2016

Revert "Adding UMA to track previews opt outs and blacklist eligibility"

> 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
> Cr-Commit-Position: refs/heads/master@{#431020}

BUG= 663940 , 647717 
TBR=ryansturm@chromium.org,asvitikine@chromium.org
NOPRESUBMIT=true
NOTRY=true

Review-Url: https://codereview.chromium.org/2491803002
Cr-Commit-Position: refs/heads/master@{#431091}

[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/BUILD.gn
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/previews_black_list.cc
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/previews_black_list.h
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/previews_black_list_unittest.cc
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/previews_io_data.cc
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/components/previews/core/previews_io_data_unittest.cc
[modify] https://crrev.com/032b6e1db3c05363d2db3d2323573ff3fad3512a/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/af5258ab4404766182e87f2c773051684c7d8c72

commit af5258ab4404766182e87f2c773051684c7d8c72
Author: ryansturm <ryansturm@chromium.org>
Date: Thu Nov 10 04:16:54 2016

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}

[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/BUILD.gn
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/previews_black_list.cc
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/previews_black_list.h
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/previews_black_list_unittest.cc
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/previews_io_data.cc
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/components/previews/core/previews_io_data_unittest.cc
[modify] https://crrev.com/af5258ab4404766182e87f2c773051684c7d8c72/tools/metrics/histograms/histograms.xml