[Password Manager] Delay expiration of LinuxBackendMigration histograms

The histograms are used for the crbug.com/829857 and a related
investigation on high error rates.

Bug: 919653
Change-Id: Ic1200071672e3d7378fa61560dbdc5c5ee3a329f
Reviewed-on: https://chromium-review.googlesource.com/c/1400686
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621085}
1 file changed