Check for type registeration of Custom element before validating options

Early return form CustomElementRegistry::registerElement, if type
is already registered, before validating options.

BUG=

Review URL: https://codereview.chromium.org/467033006

git-svn-id: svn://svn.chromium.org/blink/trunk@180837 bbb929c8-8fbe-4397-9dbb-9b2b20218538