sdch: add Sdch3.AdvertisedWithSecureScheme histogram

sdch: add Sdch3.AdvertisedWithSecureScheme histogram

This histogram tracks how frequently SDCH is advertised over a secure scheme.

BUG=488086
Committed: https://crrev.com/d540b4c265392b42bbf4f713c08775eb3f397159
Cr-Commit-Position: refs/heads/master@{#329921}

[Elly Fong-Jones, 2015-05-14 15:45:59]

ellyjones@chromium.org changed reviewers:
+ asvitkine@chromium.org, rdsmith@chromium.org

[Elly Fong-Jones, 2015-05-14 15:46:13]

rdsmith, asvitkine: PTAL? :)

[Alexei Svitkine (slow), 2015-05-14 15:48:03]

lgtm

https://codereview.chromium.org/1137753006/diff/1/tools/metrics/histograms/hi...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/1137753006/diff/1/tools/metrics/histograms/hi...
tools/metrics/histograms/histograms.xml:35252: +    Whether an SDCH dictionary
was advertised over a secure scheme or not.
Nit: Mention where it's logged.

[Randy Smith (Not in Mondays), 2015-05-14 19:37:30]

lgtm

[Elly Fong-Jones, 2015-05-14 19:37:44]

The CQ bit was checked by ellyjones@chromium.org

[Elly Fong-Jones, 2015-05-14 19:37:44]

The patchset sent to the CQ was uploaded after l-g-t-m from
asvitkine@chromium.org
Link to the patchset: https://codereview.chromium.org/1137753006/#ps20001
(title: "Add more detail for histogram")

[commit-bot: I haz the power, 2015-05-14 19:39:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1137753006/20001

[commit-bot: I haz the power, 2015-05-14 20:35:40]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-14 20:36:28]

Patchset 2 (id:??) landed as
https://crrev.com/d540b4c265392b42bbf4f713c08775eb3f397159
Cr-Commit-Position: refs/heads/master@{#329921}