Add StaleWhileRevalidate paint timing histograms.

Generally PaintTiming histograms haven't been added to histograms.xml
it appears. So this wasn't done in http://crrev.com/e5d647d0

But chirp is complaining about the missing histograms so corrrect that.

BUG=348877

Change-Id: I2605949ae729b93852147716fed6d4e15064688f
Reviewed-on: https://chromium-review.googlesource.com/c/1326802
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607046}
1 file changed