ash: Rename LauncherButtonHost to ShelfButtonHost.

- move launcher_button_host.h from launcher/ to shelf/
- rename to shelf_button_host.h

BUG=248353
TEST=None, no functional changes
R=harrym@chromium.org,jamescook@chromium.org

Review URL: https://codereview.chromium.org/57453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233337 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233337