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In addition to the at-launch reauthorization, this adds an at-update
reauthorization step. It only runs for users not on a system Keystone ticket,
because the updater runs as root when on a system ticket, and root can't read
individual user's Keychains. The at-update reauthorization is intended to
handle the reauthorization for users who rarely restart Chrome and might miss
out on the at-launch step during the window where Chrome is signed by the old
certificate but has the new reauthorization code in place. The at-update
reauthorization step can remain in place even after the certificate switch
occurs by shipping an old reauthorization stub binary signed with the old
certificate.

BUG=629906
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2745223006
Cr-Commit-Position: refs/heads/master@{#457530}