cros: Make WMModeButtonTray use SetPressedCallback

This will enable metrics!

Bug: 250632411
Change-Id: I693396184a4465a20e86fab0405dee9f7abc4acd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4054480


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076565}