Update ServiceWorker event handled status histogram
Currently ServiceWorker.UnhandledEventRatio is a bit noisy and not
very informative. This CL changes the UMA as following:
- Simplify per-worker event handled status. Probably we don't need to
record the handled ratio in percentage, but just record status in
some buckets (e.g. 'all events' or 'no events' are handled etc)
would be enough.
- Reset the metrics when the SW's stopped, so that the metrics won't be
skewed by the lifetime of versions.
- Exclude ServiceWorker stats for NTP like scope, as it tends to dominate
the stats and make the resutls largely skewed.
BUG=483354
TEST=(manually tested locally with chrome://histograms)
Review URL: https://codereview.chromium.org/1178043002
Cr-Commit-Position: refs/heads/master@{#337576}
4 files changed
