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CustomButtonStateChangedDelegate was itnroduced in http://crrev.com/159994, and
its use was removed in http://crrev.com/185369 It hasn't been used since. So
remove the code.

BUG=none
R=tfarina@chromium.org

Review URL: https://codereview.chromium.org/872473004

Cr-Commit-Position: refs/heads/master@{#313515}