Add metrics for webapk update rate.

Add metrics for webapk update rate.

Updates can fail for different reasons than install (e.g. signing issue).
We expect all installs should be for the reason 'install failed' but need
to verify.
BUG=719955
Review-Url: https://codereview.chromium.org/2867373003
Cr-Commit-Position: refs/heads/master@{#470376}
Committed: https://chromium.googlesource.com/chromium/src/+/884274a17f127dcd2f60f545262c873a7fbbdfee

[Yaron, 2017-05-09 14:38:25]

yfriedman@chromium.org changed reviewers:
+ asvitkine@chromium.org, hanxi@chromium.org

[Xi Han, 2017-05-09 14:40:42]

lgtm

[Alexei Svitkine (slow), 2017-05-09 14:54:21]

lgtm

[Yaron, 2017-05-09 14:57:49]

The CQ bit was checked by yfriedman@chromium.org

[commit-bot: I haz the power, 2017-05-09 14:58:33]

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, 2017-05-09 17:57:05]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1494341869618330, "parent_rev":
"c52544194f2bf97e72cf9215f2c3d40928929036", "commit_rev":
"884274a17f127dcd2f60f545262c873a7fbbdfee"}

[commit-bot: I haz the power, 2017-05-09 17:57:23]

Description was changed from

==========
Add metrics for webapk update rate.

Updates can fail for different reasons than install (e.g. signing issue).
We expect all installs should be for the reason 'install failed' but need
to verify.
BUG=719955
==========

to

==========
Add metrics for webapk update rate.

Updates can fail for different reasons than install (e.g. signing issue).
We expect all installs should be for the reason 'install failed' but need
to verify.
BUG=719955

Review-Url: https://codereview.chromium.org/2867373003
Cr-Commit-Position: refs/heads/master@{#470376}
Committed:
https://chromium.googlesource.com/chromium/src/+/884274a17f127dcd2f60f545262c...
==========

[commit-bot: I haz the power, 2017-05-09 17:57:24]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/884274a17f127dcd2f60f545262c...