Make the app list background corner radius based on shelf height.

This will make it easier to change shelf height for dense shelf changes and
other shelf changes moving forward.

Bug: 973483
Change-Id: Ie4c062d7c9b84c4a97f2c58b66774fa1c48eb87c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1704856
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678019}