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}
(cherry picked from commit 5c61d124a2b14a963c7169359a99a9d2e5bdf560)

Review-Url: https://codereview.chromium.org/2621413004 .
Cr-Commit-Position: refs/branch-heads/2924@{#735}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed: https://chromium.googlesource.com/chromium/src/+/7c546d237095918c4fd62dd84b78a5fdabeb3be1

[twifkak, 2017-01-11 20:47:02]

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

Review-Url: https://codereview.chromium.org/2628513002
Cr-Commit-Position: refs/heads/master@{#442637}
(cherry picked from commit 5c61d124a2b14a963c7169359a99a9d2e5bdf560)
==========

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}
(cherry picked from commit 5c61d124a2b14a963c7169359a99a9d2e5bdf560)

Review-Url: https://codereview.chromium.org/2621413004 .
Cr-Commit-Position: refs/branch-heads/2924@{#735}
Cr-Branched-From:
3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed:
https://chromium.googlesource.com/chromium/src/+/7c546d237095918c4fd62dd84b78...
==========

[twifkak, 2017-01-11 20:47:04]

Committed patchset #1 (id:1) manually as
7c546d237095918c4fd62dd84b78a5fdabeb3be1.