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This clears the controller in the OnClose() method to prevent a crash
in the event that the OnClose() callback runs more than once before the
animation is complete. This surfaced as a crash when sending a file.

Fixed: 1150176
Change-Id: I13aa8eaf3bf3f5e382d31376825f647ab287a7dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548373
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#829050}