Add a value to PasswordManager.KeychainMigration.Status histogram.

It's a follow-up to https://codereview.chromium.org/2729853005/ where MigrationStatus was extended.

BUG=595468

Review-Url: https://codereview.chromium.org/2731323002
Cr-Commit-Position: refs/heads/master@{#454939}
1 file changed