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Instead of relying on an implementation of IdentityManager::Observer and
a private flag to know when a RefreshToken is available before requesting
using PrimaryAccountAccessTokenFetcher, use PAATF's mode to wait until the
primary account has a refresh token available (i.e. kWaitUntilAvailable)
whenever needed when TokenHandleFetcher::BackfillToken() is invoked, or
simply using kImmediate if such refresh token is already available.

Bug: 905689
Change-Id: Idf8e8b58a9bda518f2bd11d140b4e8776194a8f4
Reviewed-on: https://chromium-review.googlesource.com/c/1343264
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611079}