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Adding a call back to the view controller when being popped.

Change-Id: Ib2b3800b3204ae52fd437681b4362a25682d15ec
Reviewed-on: https://chromium-review.googlesource.com/c/1425608
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625250}