service worker: Record some fetch event timings up to 3 minutes

Before this CL, all histograms under ServiceWorker.LoadTiming category
were recorded by UMA_HISTOGRAM_TIMES macro. The macro can record up to
10 seconds but we've realized that 10 seconds limit isn't enough for
some milestones. Use UMA_HISTOGRAM_MEDIUM_TIMES for these histograms
so that they can be recorded up to 3 minutes.

Bug: 877972
Change-Id: Icca1ddd583c87c4dd9a6ebd924b81edd2648b2f0
Reviewed-on: https://chromium-review.googlesource.com/c/1350433
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611474}
4 files changed