Make notification buttons draw a background so text gets subpixel AA.

Adds a new NSButton subclass that draws its own background before
rendering itself in the usual way.

BUG=335794

Review URL: https://codereview.chromium.org/131453005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249104 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249104