Remove TreatNullAs=NullString for HTMLFontElement

http://www.whatwg.org/specs/web-apps/current-work/multipage/obsolete.html#htmlfontelement

color has [TreatNullAs=EmptyString] in the spec, but face/size do not.

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

IE11 reflects size as the empty string.

BUG=310298

Review URL: https://codereview.chromium.org/61643028

git-svn-id: svn://svn.chromium.org/blink/trunk@162020 bbb929c8-8fbe-4397-9dbb-9b2b20218538