Remove TreatNullAs=NullString for Element.id and .className

http://dom.spec.whatwg.org/#interface-element

The tests for the updated properties FAIL in Firefox Nightly, IE11
Release Preview and Opera 12.16, with the exception of the id test
in Firefox.

This is because id and className are not on the Element interface
in these browsers, if the test is modified to create a HTMLElement
then tests for both attributes pass in all browsers.

BUG=310298

Review URL: https://codereview.chromium.org/104263002

git-svn-id: svn://svn.chromium.org/blink/trunk@163182 bbb929c8-8fbe-4397-9dbb-9b2b20218538