mash: Convert all of //ash/system to use WmShelf

This breaks dependencies on //ash/shelf in favor of one on //ash/common.

* Add WmShelf::ForPrimaryDisplay().
* Remove AshTestBase::GetPrimaryShelf() in favor of above.
* Skip unnecessary setting of shelf visibility state and auto-hide state in tests.
* Remove unnecessary includes.

BUG=615502,619636
TEST=ash_unittests

Review-Url: https://codereview.chromium.org/2093283002
Cr-Commit-Position: refs/heads/master@{#402257}
22 files changed