Have positionInParentBeforeNode() / positionInParentAfterNode() take a reference

Have positionInParentBeforeNode() / positionInParentAfterNode() take a
reference as the implementation expected the pointer to be non-null. This makes
the code look safer.

R=adamk@chromium.org, adamk

Review URL: https://codereview.chromium.org/182383012

git-svn-id: svn://svn.chromium.org/blink/trunk@168420 bbb929c8-8fbe-4397-9dbb-9b2b20218538