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The dialog will be shown for each new note taking app when it's
launched from the lock screen. The dialog will not be shown for the
same app again if the user closes it.
This adds a user pref to track note taking apps for which toast
dialog has been shown (and dismissed).

The dialog is implemented as a sysmte modal bubble dialog in
ToastDialogView.
The cl introduces FirstAppRunToastManager, which is used by
lock_screen_apps::StateController to delegate showing of the toast
dialog - the FirstAppRunToastManager, if the toast hasn't been shown
before, observe a created app window's visibility, and show the toast
dialog when the app window is shown. Once the dialog is dismissed by
the user, the FirstAppRunToastManager will update the user preferences
accordingly, to prevent dialog shows on subsequent app launches.

BUG=767717

Change-Id: I87032afb099b7ef027d82bcaa06d16a5957c3527
Reviewed-on: https://chromium-review.googlesource.com/720099Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509562}