Make page switch on drag happen at top and bottom for experimental app list.

This CL makes dragging an app in the experimental app list change pages at
the top and bottom of the apps grid rather than at the sides. This matches
the scroll behavior of the experimental app list.

BUG=406222

Review URL: https://codereview.chromium.org/518673008

Cr-Commit-Position: refs/heads/master@{#293673}