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This is done in similar fashion to what was done with CaptionBar to
decouple the controller and event callbacks:
https://chromium-review.googlesource.com/c/chromium/src/+/1086508/6

Bug: b:80542452
Change-Id: I992d2f5d0ee63e4d28a383b4e4f7ceafeabfff80
Reviewed-on: https://chromium-review.googlesource.com/1088233Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#565392}