Replace Position::anchorNode()->document() to Position::document()

This patch replaces Position::anchorNode()->document() to Position::document(). Implementation of Position::document() is identical to Position::anchorNode()->document().

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

Review URL: https://codereview.chromium.org/23494042

git-svn-id: svn://svn.chromium.org/blink/trunk@157511 bbb929c8-8fbe-4397-9dbb-9b2b20218538