AppListController refactoring part 2: Ash's AppListShowerDelegate imlementation.

Part 1 of the refactoring: https://crrev.com/1830293002
Design doc: https://docs.google.com/document/d/1M9vqTTuprMssRXs8UIVaFjQUq3lUxSzwrWoPjPvca6Q/edit?ts=56d0e9f0#heading=h.b2znwtuxd2x2

This CL adds ash::AppListShowerDelegate, which is the Ash implementation of
app_list::AppListShowerDelegate. At this point it is unused - Ash is still using
the old ash::AppListController class to control the app list.

BUG=557408

Review URL: https://codereview.chromium.org/1856943003

Cr-Commit-Position: refs/heads/master@{#386123}
8 files changed