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This change removes the legacy close behavior of DialogDelegate,
which was as follows: if a dialog was closed without either
accepting or cancelling, if the dialog had only an ok button, the
Accept() method would be called; otherwise, the Cancel() method
would be called.

This behavior was only depended on by a single dialog - the
collected cookies dialog - so this change migrates the collected
cookies dialog over to the new callback system.

This change will allow for introducing a useful invariant: that the
DialogDelegate::Close() callback always runs after
WidgetObservers observe that the dialog has begun to close.

Bug: 1085949
Change-Id: I214140a9cab687be4c36fc3961089d231f3cf1be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337342
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795056}