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This adds support for starting Sync-the-transport even when there is no
primary (aka signed-into-Chrome aka blessed-for-Sync-the-feature)
account.
This is implemented by SyncAuthManager listening for OnAccountsInCookieUpdated,
and using the first cookie account if there is no primary account and we have
a refresh token for the first cookie account. This also makes it necessary to
cache the current Sync account, so that we can detect changes to it.

IsSyncFeatureEnabled now also checks that the current account is the primary
one, so that we don't accidentally enable full Sync for a secondary account.

Bug: 871221
Change-Id: I07c7db04cc92e9a40126c89c0fcfea49e03117d1
Reviewed-on: https://chromium-review.googlesource.com/1162174
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587082}