Report Login.SuccessReason in LoginPerformer

Login.SuccessReason already exists in histograms.xml but was not used.

Do not report offline and online login success separately as I think it's
unnecessary.

BUG=175906

Review-Url: https://codereview.chromium.org/2842083002
Cr-Commit-Position: refs/heads/master@{#467194}
2 files changed