Remove TreatNullAs=NullString for HTMLHtmlElement

http://whatwg.org/html#HTMLHtmlElement-partial

The manifest attribute is not in the spec, so leave it alone pending a
decision about what to do with it:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/dSTAYLhhV3o/2Sh66-B7oAIJ

The test passes in Firefox Nightly, IE11 Release Preview and Opera 12.16.

BUG=310298

Review URL: https://codereview.chromium.org/101153003

git-svn-id: svn://svn.chromium.org/blink/trunk@163082 bbb929c8-8fbe-4397-9dbb-9b2b20218538