Remove ShelfDelegate, subclasses, accessors, etc.
Move ShelfDelegate responsibilities to ash::ShelfModel, add tests.

Move ChromeLauncherControllerImpl ownership to ChromeShellDelegate.
Add lifetime management functions: ShelfInit() and ShelfShutdown().
Apply similar ownership changes in tests; expose an AshTestHelper.

Remove ShelfDelegate responsibilities from ChromeLauncherController:
* Apply chrome-specific item info/delegates in ShelfItemAdded.
  - Support ShelfModel::SetItem calls amid ShelfItemAdded in ShelfView.
  - Register some item delegates early to avoid a default one.
* Cleanup chrome-side item info in ShelfItemRemoved.

TODO: Make ChromeLauncherControllerImpl lifetime more flexible.
TODO: Move functions to (or add accessors on) ash::[mojom::]ShelfController.

BUG=557406,698887
TEST=No Chrome OS shelf behavior changes.
R=jamescook@chromium.org,sky@chromium.org

Review-Url: https://codereview.chromium.org/2791463002
Cr-Commit-Position: refs/heads/master@{#465821}