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- Only MarkSinglePopupAsShown of already shown pup-ups;
- Make LoginStateNotificationBlocker to defer showing notifications
  until the active user PrefService is initialized. This allows
  Shelf to apply user prefs first so that WebNotificationTray does
  not HidePopupBubble when user PrefService is initialized sometime
  later after session state changes to active;

Bug: 721717
Change-Id: I86e9978be7b56e92ce847f0086a5fa2e4f6b72db
Reviewed-on: https://chromium-review.googlesource.com/782359
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518406}