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Instead of using deferred initialization, show the account selector when
a scene becomes active. This means if there are multiple scenes, the
user will always have to go through this dialog even if they switch to
another window.
Also completely get rid of AppNavigation, as it's not used anymore.

Bug: 1045663
Change-Id: Idbbd19f1e188b033e39ce318def6b1338ad3ac6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107581
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751982}