Introduce AccessibiltyDelegate

I'll incorporate changes in https://codereview.chromium.org/27238008 when it's landed.

BUG=none

Review URL: https://codereview.chromium.org/37663004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230619 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230619