Remove TreatNullAs=NullString for HTMLHRElement

http://whatwg.org/html#HTMLHRElement-partial

Also add the missing color attribute.

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

IE throws an exception for align and reflects color/size/width as
the empty string.

Opera does not support the color attribute.

BUG=310298

Review URL: https://codereview.chromium.org/98723003

git-svn-id: svn://svn.chromium.org/blink/trunk@162989 bbb929c8-8fbe-4397-9dbb-9b2b20218538