CrOS: Create ShelfButtonDelegate as an interface for ShelfButton

To enable the refactorings which will enable the new shelf, remove
references to ShelfView from ShelfButton, ShelfControlButton, and
classes that extend ShelfControlButton.

Some ShelfButtons (ShelfAppButton) will keep their ShelfView references
because ShelfView will remain their host view.

Change-Id: Iefbdadb05f1d2275e274dace6551905d17104876
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678442
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673887}

• ash/BUILD.gn[diff]
• ash/shelf/assistant_overlay.cc[diff]
• ash/shelf/back_button.cc[diff]
• ash/shelf/back_button.h[diff]
• ash/shelf/default_shelf_view.cc[diff]
• 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/ink_drop_button_listener.h[Deleted - diff]
• ash/shelf/kiosk_next_shelf_view.cc[diff]
• ash/shelf/overflow_button.cc[diff]
• ash/shelf/overflow_button.h[diff]
• ash/shelf/shelf_app_button.cc[diff]
• ash/shelf/shelf_app_button.h[diff]
• ash/shelf/shelf_button.cc[diff]
• ash/shelf/shelf_button.h[diff]
• ash/shelf/shelf_button_delegate.h[Added - diff]
• ash/shelf/shelf_control_button.cc[diff]
• ash/shelf/shelf_control_button.h[diff]
• ash/shelf/shelf_view.cc[diff]
• ash/shelf/shelf_view.h[diff]