Use more references in ContainerNode code

Use more references in ContainerNode code instead of pointers when they cannot
be null. This makes the code look safer and gets rid of unnecessary null
checks.

R=adamk

Review URL: https://codereview.chromium.org/54273007

git-svn-id: svn://svn.chromium.org/blink/trunk@161202 bbb929c8-8fbe-4397-9dbb-9b2b20218538