Convert AvatarLabel from TextButton to LabelButton.

TextButton will be deprecated soon; use LabelButton.
Use views::Border as the AvatarLabelBorder base class.
Other minor cleanup.

BUG=155363
TEST=No regressions of this Windows/CrOS/LinuxAura supervised user UI.
R=sky@chromium.org

Review URL: https://codereview.chromium.org/163953006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251241 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251241