Add UMA for service worker update checking

Currently we don't have UMAs only for update checking. As we are going to enable
ServiceWorkerImportedScriptUpdateCheck, we want to monitor if it's working
correctly. This CL adds two metrics for that purpose:

- ServiceWorker.UpdateCheck.Result
- ServiceWorker.UpdateCheck.UpdateFound

Bug: 984855
Change-Id: I0e3b3489d58fbe7640cb27893ae11d81f93d3e58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1705201
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Auto-Submit: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679479}
5 files changed