GitLab

The histograms were updated to record based on random sampling. Update
the description to specify this. Also update the expiry dates of
memory histograms.

BUG=1117570

Change-Id: Ia03d9f6b598bc832aa7411fbc27e427c1436a968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2381065
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809099}