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This is the first step of an effort to de-duplicate some code between
ShelfButton, AppListButton, BackButton, ShelfControlButton.

This makes the class inheritance of ShelfControlButton a lot closer to
ShelfButton. None of the functionality of ImageButton is used anyway,
so inheriting from plain Button directly makes more sense.

Bug: 714781
Change-Id: I20cdc87b39a82ea70813ca7492df79890a255126
Reviewed-on: https://chromium-review.googlesource.com/c/1395565Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619992}