Add Blink.ResourceFetcher.StaleWhileRevalidateDuration histogram.

This histogram tracks the duration of completed stale revalidation
attempts. This value will be used as a rough approximation of
the benefit received by using Stale While Revalidate.

BUG=348877

Change-Id: I47601e667394c401ccdc96afc4d73d6c464faa3b
Reviewed-on: https://chromium-review.googlesource.com/1133961
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574564}
3 files changed