Have ElementTraversal / NodeTraversal's next() methods take a reference

Have ElementTraversal / NodeTraversal's next() methods take a reference instead
of a pointer and the argument is expected to be non-null. This makes the code
look safer.

R=adamk

Review URL: https://codereview.chromium.org/67473002

git-svn-id: svn://svn.chromium.org/blink/trunk@161835 bbb929c8-8fbe-4397-9dbb-9b2b20218538