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This is more readable to me (after using it twice) and you don't need to
remember how to spell ui::DIALOG_BUTTON_CANCEL.

It also removes Set{Accept,Cancel}Callback by making these callbacks
part of Add{Ok,Cancel}Button().

Bug: 1106422
Change-Id: I208b80c041616ac506e4b77913a7bb6150c200dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351370
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797479}