Remove DOMSelection::setPosition

This method shares the same implementation as DOMSelection::collapse, so make
the bindings use that one instead.

Review URL: https://codereview.chromium.org/124193003

git-svn-id: svn://svn.chromium.org/blink/trunk@164631 bbb929c8-8fbe-4397-9dbb-9b2b20218538