Let Node.nodeName never return null

http://dom.spec.whatwg.org/#dom-node-nodename

Since I could not find any case where it would currently return null, no
observable diference is expected and no tests were added.

Also remove an outdated comment.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/105733008

git-svn-id: svn://svn.chromium.org/blink/trunk@163269 bbb929c8-8fbe-4397-9dbb-9b2b20218538