Add Precache.CacheStatus.Prefetch UMA.

Add Precache.CacheStatus.Prefetch UMA.

This is the complement to the Precache.CacheStatus.NonPrefetch UMA. It
helps us to answer oft-asked questions about where the service is
providing the most benefit.

BUG=679543
Review-Url: https://codereview.chromium.org/2628513002
Cr-Commit-Position: refs/heads/master@{#442637}
Committed: https://chromium.googlesource.com/chromium/src/+/5c61d124a2b14a963c7169359a99a9d2e5bdf560

[twifkak, 2017-01-10 00:31:49]

twifkak@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Alexei Svitkine (slow), 2017-01-10 16:16:32]

lgtm

[twifkak, 2017-01-10 17:31:11]

The CQ bit was checked by twifkak@chromium.org

[commit-bot: I haz the power, 2017-01-10 17:31:31]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-10 18:34:42]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1484069471032680, "parent_rev":
"bc0eecbb3bca9a35b72e4dc926a7cfc635b9ef7a", "commit_rev":
"5c61d124a2b14a963c7169359a99a9d2e5bdf560"}

[commit-bot: I haz the power, 2017-01-10 18:35:11]

Description was changed from

==========
Add Precache.CacheStatus.Prefetch UMA.

This is the complement to the Precache.CacheStatus.NonPrefetch UMA. It
helps us to answer oft-asked questions about where the service is
providing the most benefit.

BUG=679543
==========

to

==========
Add Precache.CacheStatus.Prefetch UMA.

This is the complement to the Precache.CacheStatus.NonPrefetch UMA. It
helps us to answer oft-asked questions about where the service is
providing the most benefit.

BUG=679543

Review-Url: https://codereview.chromium.org/2628513002
Cr-Commit-Position: refs/heads/master@{#442637}
Committed:
https://chromium.googlesource.com/chromium/src/+/5c61d124a2b14a963c7169359a99...
==========

[commit-bot: I haz the power, 2017-01-10 18:35:12]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/5c61d124a2b14a963c7169359a99...