ServiceWorker: Add UMA for Activate and Install event results.

ServiceWorker: Add UMA for Activate and Install event results.

We want to change activate so that rejecting the waitUntil promise
is the same as resolving it. Adding UMA to get an idea of the impact
of the change. It could be useful to have this data anyway, so also
adding it for Install.

BUG=480050
Committed: https://crrev.com/375d6d1dfc251498b878c71574a24494bdb18216
Cr-Commit-Position: refs/heads/master@{#326925}

[falken, 2015-04-24 03:11:23]

falken@chromium.org changed reviewers:
+ nhiroki@chromium.org

[falken, 2015-04-24 03:11:24]

ptal

[falken, 2015-04-24 04:00:13]

The CQ bit was checked by falken@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-24 04:00:21]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1098953005/20001

[nhiroki, 2015-04-24 04:01:33]

lgtm

[falken, 2015-04-24 04:05:25]

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

[falken, 2015-04-24 04:05:26]

+asvitkine for histograms

[commit-bot: I haz the power, 2015-04-24 05:00:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-24 05:00:32]

Dry run: This issue passed the CQ dry run.

[Alexei Svitkine (slow), 2015-04-24 16:06:30]

lgtm

[falken, 2015-04-24 23:03:31]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2015-04-24 23:04:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1098953005/20001

[commit-bot: I haz the power, 2015-04-24 23:05:04]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-24 23:06:24]

Patchset 2 (id:??) landed as
https://crrev.com/375d6d1dfc251498b878c71574a24494bdb18216
Cr-Commit-Position: refs/heads/master@{#326925}