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The lifetime of CrostiniExportImportNotificationController used to be
1to1 with the lifetime of the NotificationUI. This is unnecessary and
makes it harder to reuse this functionality more broadly (see
https://chromium-review.googlesource.com/c/chromium/src/+/1985621/3 for
an example which has very complicated lifetime management due to this).

This CL separates the Controller's lifetime from the UI's lifetime by
extracting the Controller's UI event handling logic (clicking/closing)
into a delegate which the controller creates, such that its lifetime
is 1to1 with the UI, thus allowing the Controller to be destroyed at
anytime without affecting the behaviour of the UI.

Bug: 1024693
Change-Id: I6c67016f862e80dcd1c92a12d507f484c28fe4de
tbr: stevenjb@chromium.org
tbr: tengs@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991060
Commit-Queue: Julian Watson <juwa@google.com>
Reviewed-by: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#731788}