Remove unused class VisiblePositionRange

This patch removes unused class VisiblePositionRange from AXObject.h

BUG=n/a
TEST=n/a; no behavior changes
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/36523002

git-svn-id: svn://svn.chromium.org/blink/trunk@160303 bbb929c8-8fbe-4397-9dbb-9b2b20218538