Have NodeWithIndex deal with references instead of pointers

Have NodeWithIndex deal with references instead of pointers. This makes the
code look safer.

R=adamk@chromium.org, adamk, rwlbuis

Review URL: https://codereview.chromium.org/176863009

git-svn-id: svn://svn.chromium.org/blink/trunk@168418 bbb929c8-8fbe-4397-9dbb-9b2b20218538