Drop traverseNextElement() method from HTMLCollection class

Drop traverseNextElement() method from HTMLCollection class to avoid code
duplication with traverseForwardToOffset(). This method was only used
by supportedPropertyNames() and updateIdNameCache(). Now these methods
uses the standard traversal API (HTMLCollection::length() / item(index)),
and thus the standard code path for HTMLCollection traversal.

I did not see any regression with this change on Dromaeo:
http://dromaeo.com/?id=223149,223150

R=adamk@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/336133002

git-svn-id: svn://svn.chromium.org/blink/trunk@176253 bbb929c8-8fbe-4397-9dbb-9b2b20218538