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Returning transient errors when the refresh token isn't loaded yet was
better than persistent errors, but it could result in quite a few errors
being returned before a success, sometimes causing DriveFS to give up
anyway. Avoid this by waiting for the account to be ready before
servicing any access token requests.

Bug: 855002
Change-Id: I1578f8f9116592e1ee2864a804269a4a49ba8c92
Reviewed-on: https://chromium-review.googlesource.com/1125551Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572482}