[Password Manager] Add PasswordManager.LinuxBackendMigration.AttemptResult

This metric is reported at the end of an attempt to migrate away from the
Linux native backends and into LoginDatabase.

This complements the PasswordManager.LinuxBackendMigration.Adoption, which
is reported on the next run and can see crashed migrations but can't
contribute successful migration pings to the finch experiment histograms.

Bug: 571003
Change-Id: I6255ffdd129be423dadaad3ba00e3751e9fea81b
Reviewed-on: https://chromium-review.googlesource.com/c/1412932
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623212}
3 files changed