CrOS Shelf: Make the home button more independent

This makes the home button its own shelf button delegate, so it knows
how to handle its own events and how to pass on focus when needed.

Bug: 973443
Change-Id: Iae47147624eb934fa6a8caca8e4691a7268c257b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707899
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678845}

• ash/shelf/home_button.cc[diff]
• ash/shelf/home_button.h[diff]
• ash/shelf/home_button_controller.cc[diff]
• ash/shelf/home_button_controller.h[diff]
• ash/shelf/shelf_button.h[diff]
• ash/shelf/shelf_view.cc[diff]