Add StaleWhileRevalidateExperiment histograms.

Add histograms
PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2.NavigationToLoadEventFired,
PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2.NavigationToFirstLayout,
and
PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2.NavigationToFirstTextPaint.

These behave the same as the base histograms, but they are only recorded
for a subset of domains which have render-blocking resources served with
the Cache-Control stale-while-revalidate directive.

These histograms permit us to measure the benefit in load time from the
implementation of the directive wihout noise from domains which don't
use it.

This CL was split from the original CL https://codereview.chromium.org/1303973009/
with changes to the filenames and target names in //net.

BUG=348877
TEST=manual

Review URL: https://codereview.chromium.org/1433893002

Cr-Commit-Position: refs/heads/master@{#361549}
14 files changed