[custom-elements] Added element registration fix

Description:
  Forced document.register & document.registerElement
  to be recognized as the same function as long as one
  exists. registerElement is the latest name update in
  the custom elements. W3C has a bug on this and registerElement
  is currently in Chrome Canary. Canary throws errors
  when register is called.

W3C Bug:

  http://lists.w3.org/Archives/Public/public-webapps/2013OctDec/1021.html