Remove TreatNullAs=NullString for miscellaneous HTML elements

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

IE11 throws an exception for all except HTMLQuoteElement.cite.

BUG=310298

Review URL: https://codereview.chromium.org/62083003

git-svn-id: svn://svn.chromium.org/blink/trunk@161535 bbb929c8-8fbe-4397-9dbb-9b2b20218538