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This change:
1) Adds an inner WidgetObserver subclass to Button to allow Button to observe
   its Widget's activation state changing;
2) Introduces a notion of "visual state" to buttons, as distinct from their
   existing state; the visual state governs how a button is drawn.
3) Has LabelButton use the visual state to decide which image to use instead
   of the raw state.

The end result is that when a LabelButton's Widget deactivates, that LabelButton
will take on a disabled appearance.

Bug: 848593
Change-Id: If5bd135c254fd36669e76fd39338dc24387602aa
Reviewed-on: https://chromium-review.googlesource.com/c/1251330
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600471}