[WebUI] Better focus ring drawing.

Ensure that focus rings do not change width when the currently focused
button becomes disabled (due to action triggered by button)

BUG=321928
R=estade@chromium.org

Review URL: https://codereview.chromium.org/149603004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249210 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249210