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This makes views::Combobox use a base::RepeatingCallback internally.
Pre-existing uses of ComboboxListener are wrapped in a callback in
set_listener(). Cleanups removing ComboboxListeners will follow.

Bug: 772945
Change-Id: I2c1b4a7637ba5125fd7774e1f18f4751ced9959c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401951
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805493}