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Considers unknown signin status as a valid status when browser is added (For mac, it's possible to have chrome running with no browser window open, which should be signin-status-unknown).

Collects more stats when getting signin status. (record it in histograms whenever any error is seen.)

BUG=412794

Review URL: https://codereview.chromium.org/554863002

Cr-Commit-Position: refs/heads/master@{#294192}