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|ChromeOSOAuth2TokenServiceDelegate| loads tokens at startup and calls
|FireRefreshTokensLoaded| before some token service observers may have
had a chance to register themselves. This causes them to miss the
|OnRefreshTokensLoaded| callback.

For now, the solution is to respect the temporal coupling between
|OAuth2TokenServiceDelegate::LoadCredentials| and
|OAuth2TokenService::AddObserver|, i.e. |LoadCredentials| is called
only when all observers have been added. Ideally, it should be an
observer's responsibility to check whether it has been initialized 'too
late' and may have missed the |OnRefreshTokensLoaded| notification.

Bug: 820046
Change-Id: I3ac67798d165afe230fa29e03ec7af75a9a093a9
Reviewed-on: https://chromium-review.googlesource.com/1069353
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561422}