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This allows an optional callback to be notified whenever the user
completes or cancels the wizard.

Bug: 1015201
Change-Id: I93cfa973518b40a840dc20068de5e44636b3806a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1907769
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714371}