Stop passing the root node to node lists traversal functions

Stop passing the root node to node lists traversal functions. Their
implementation can call LiveNodeListBase::rootNode() if needed and do not
really need it to be passed in argument.

Review URL: https://codereview.chromium.org/224303004

git-svn-id: svn://svn.chromium.org/blink/trunk@170800 bbb929c8-8fbe-4397-9dbb-9b2b20218538