Remove TreatNullAs=NullString for HTMLButtonElement

http://whatwg.org/html#htmlbuttonelement 

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

IE11 returns an empty string for HTMLButtonElement.type.

Leave setCustomValidity until later, since it appears on several interfaces.

BUG=310298

Review URL: https://codereview.chromium.org/59903003

git-svn-id: svn://svn.chromium.org/blink/trunk@161455 bbb929c8-8fbe-4397-9dbb-9b2b20218538