Remove TreatNullAs=NullString for HTMLModElement

http://whatwg.org/html#htmlmodelement

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16.

BUG=310298

Review URL: https://codereview.chromium.org/99213002

git-svn-id: svn://svn.chromium.org/blink/trunk@162997 bbb929c8-8fbe-4397-9dbb-9b2b20218538