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This allows clients to supply a callback upon starting a promo. When
the bubble closes, it will be called.

No information is given in this CL. In the future, supplying a close
reason is desirable.

Bug: 1143465
Change-Id: I6a1af8d6f314cc268e67406371a5414cbc4feb6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508395Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822789}