Fix check-webkit-style errors in core/accessibility.

This is just to make future refactoring easier, since check-webkit-style
makes you fix any style errors when you modify a file in any way.

BUG=none

Review URL: https://codereview.chromium.org/27116005

git-svn-id: svn://svn.chromium.org/blink/trunk@159785 bbb929c8-8fbe-4397-9dbb-9b2b20218538