Remove TreatNullAs=NullString for HTMLSelectElement

http://whatwg.org/html#htmlselectelement

The value attribute returns "" when there is no selected child
<option> element, so add another test to verify that null
stringifies to "null" on setting.

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/101573002

git-svn-id: svn://svn.chromium.org/blink/trunk@163087 bbb929c8-8fbe-4397-9dbb-9b2b20218538