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Print Preview Handler was only observing the identity manager if
mirror was enabled. However, mirror is now generally disabled, so Print
Preview never actually received any notifications about cookie jar
changes. This CL changes PrintPreviewHandler to always add this observer
unless cloud print is disabled (in which case Print Preview does not
need login information).

With the observer, the callback after sign in completion is no longer
necessary. If the user signs in to cloud print successfully,
IdentityManager::Observer::OnAccountToCookieCompleted() will be called.
The reload-printer-list event fired from this function triggers a call
to onDestinationsReload(), which is the same call that happens after
the sign in promise is resolved.

The only expected behavior change from this CL is that if the
destinations dialog is open when a user signs in or out in a different
tab, the destinations should refresh.

Bug: 932692, 958767
Change-Id: Ibdabf382f4eb8f6c438376b2f2415c27f4e9c586
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626933Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663859}