Make CharacterData.appendData() argument mandatory

Make CharacterData.appendData() argument mandatory as per the latest specification:
http://dom.spec.whatwg.org/#characterdata

This behavior is consistent with IE10 and Firefox.

BUG=266351

Review URL: https://chromiumcodereview.appspot.com/21364003

git-svn-id: svn://svn.chromium.org/blink/trunk@155323 bbb929c8-8fbe-4397-9dbb-9b2b20218538