Add experiment for Stale While Revalidate for whitelisted hosts.

Add metrics to to track the time taken first contentful paint
for pages that use whitelisted hosts for stale while revalidate.
This breaks the timings into whether the resource was loaded from
the network, cache, stale-cache loads.

BUG=348877

Change-Id: I4e710d6f52517a815d68d478584818501c10e0ea
Reviewed-on: https://chromium-review.googlesource.com/c/1297461
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604922}
20 files changed