Convert OAuthTokenFetchedCallback to as OnceCallback<>

The callback is only invoked once, so change the type from
base::Callback (which is an alias for RepeatingCallback) to
base::OnceCallback.

Bug: none
Change-Id: I88459dc79073fe890645aae5457678172d1bfc53
Reviewed-on: https://chromium-review.googlesource.com/c/1344142
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610430}
9 files changed