Remove TreatNullAs=NullString for various HTML elements

http://whatwg.org/html#htmllabelelement
http://whatwg.org/html#HTMLLegendElement-partial
http://whatwg.org/html#htmlmapelement
http://whatwg.org/html#htmloptgroupelement
http://whatwg.org/html#htmlstyleelement
http://whatwg.org/html#htmltitleelement

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 HTMLLegendElement.align.

BUG=310298

Review URL: https://codereview.chromium.org/99103002

git-svn-id: svn://svn.chromium.org/blink/trunk@163009 bbb929c8-8fbe-4397-9dbb-9b2b20218538