Keep Signin.AccountTracker.* histograms

Bump expiration date of Signin.AccountTracker.CountOfLoadedAccounts
and Signin.AccountTracker.GaiaIdMigrationState by four releases (or
about six month).

Bug: 960028
Change-Id: Ie168fb5cc5b972555a92e41073c34679900037bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634741
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664268}
1 file changed