Have LiveNodeList::rootNode() return a reference

Have LiveNodeList::rootNode() return a reference as it can never
return null. This makes the calling code look safer.

BUG=

Review URL: https://codereview.chromium.org/29873003

git-svn-id: svn://svn.chromium.org/blink/trunk@160143 bbb929c8-8fbe-4397-9dbb-9b2b20218538