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The linked bug uncovered a case where ShowClientNotification was called
multiple times with the same notification (i.e. the same notification
ID) on the Chrome side, while Ash sent a HandleNotificationClosed
for the first shown notification which arrived in Chrome after the
second ShowClientNotification. Then Ash subsequently sent a second
HandleNotificationClosed, but since the notification had already been
removed from the Chrome-side cache in response to the first
HandleNotificationClosed, an error was thrown (specifically, the check
in NotificationPlatformBridgeChromeOs::HandleNotificationClosed()).

The solution is to create a token that is separate from the
notification's ID to identify the Display call to which the Close is
paired. This token is not used for other callbacks, such as the button
handlers, because then Chrome could drop user input that occurred around
the same time as a notification being updated (seems fairly likely for
any notification that has a progress bar, such as downloads).

Bug: 825141
Change-Id: I01f66dcc1107bfa7337c447780edb897e24498e6
Reviewed-on: https://chromium-review.googlesource.com/979268
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548484}