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Also replaces ButtonController::OnStateChanged() with a property change
callback.

This will allow removing ButtonObserver entirely in a future CL, and
potentially all Button::StateChanged() overrides as well.

Bug: none
Change-Id: I143a6e7d0668d8bee7535f3d57bbf04430685428
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284218
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785940}