Have ContainerNodeAlgorithms deal with references instead of pointers

      Have ContainerNodeAlgorithms deal with references instead of pointers
since those can never be null. This makes the code look safer.

R=adamk

Review URL: https://codereview.chromium.org/49153006

git-svn-id: svn://svn.chromium.org/blink/trunk@160769 bbb929c8-8fbe-4397-9dbb-9b2b20218538