CharacterData.substringData() arguments should be mandatory

CharacterData.substringData() arguments should be mandatory as per the
specification:
http://dom.spec.whatwg.org/#dom-characterdata-substringdata

Also, we should not throw if the second argument (count) is negative.

This behavior is consistent with IE10 and Firefox.

BUG=237739

Review URL: https://chromiumcodereview.appspot.com/21675003

git-svn-id: svn://svn.chromium.org/blink/trunk@155516 bbb929c8-8fbe-4397-9dbb-9b2b20218538