Rename LiveNodeList::nodeMatches() to elementMatches()

Rename LiveNodeList::nodeMatches() to elementMatches() as LiveNodeList only
deals with Element items now.

R=esprehn, adamk

Review URL: https://codereview.chromium.org/198443003

git-svn-id: svn://svn.chromium.org/blink/trunk@169163 bbb929c8-8fbe-4397-9dbb-9b2b20218538