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This CL adds an API to OAuth2TokenServiceDelegate::Observer to
listen to changes in auth errors.

AccountReconcilor uses this API to trigger a ListAccount call
when a token becomes invalid.
This is necessary because in that case, the account can be invalid
in the cookie without the cookie changing. Chrome needs to make a
call to ListAccount to have an accurate view of the Gaia accounts in
the cookie.

Bug: 823714
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ie22226adfe6c425acaffe875a314c4a6b8de5e88
Reviewed-on: https://chromium-review.googlesource.com/986268
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549493}