Clean up forward declarations in Source/core/accessibility

These forward references are not needed anymore. After
this patch, Source/core/accessibility has no unneeded
forward references anymore.

Review URL: https://codereview.chromium.org/626133002

git-svn-id: svn://svn.chromium.org/blink/trunk@183218 bbb929c8-8fbe-4397-9dbb-9b2b20218538