Move some histogram recordings to ServiceWorkerStorage

This CL removes ServiceWorkerMetrics dependency from
ServiceWorkerStorage by embedding UMA recordings into it.
ServiceWorkerStorage will be moved to //components/services/storage
but ServiceWorkerMetrics will remain //content. This means that
ServiceWorkerStorage cannot use ServiceWorkerMetrics.

This CL also applies histogram guidelines
(//tools/metrics/histograms/README.md).

Bug: 1016064
Change-Id: I79ba99c096fc422166b0f0f23d27728804701113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394899
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804710}