Remove TreatNullAs=NullString for HTMLTable*Element

http://whatwg.org/html#HTMLTableCaptionElement-partial
http://whatwg.org/html#htmltablecellelement
http://whatwg.org/html#HTMLTableCellElement-partial
http://whatwg.org/html#htmltableheadercellelement
http://whatwg.org/html#HTMLTableDataCellElement-partial
http://whatwg.org/html#HTMLTableColElement-partial
http://whatwg.org/html#HTMLTableElement-partial
http://whatwg.org/html#HTMLTableRowElement-partial
http://whatwg.org/html#HTMLTableSectionElement-partial

The spec has HTMLTableDataCellElement and HTMLTableHeaderCellElement
interfaces which we don't have yet, but ignore that here.

HTMLTable*Element.bgColor and HTMLTableElement.cellPadding/cellSpacing
have [TreatNullAs=EmptyString] in the spec.

HTMLTableCellElement.headers is a DOMSettableTokenList in the spec, so
leave it alone until that is fixed.

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

IE throws an exception for the align/vAlign/frame/rules, and reflects
border/width/height as the empty string.

BUG=310298

Review URL: https://codereview.chromium.org/101493002

git-svn-id: svn://svn.chromium.org/blink/trunk@163081 bbb929c8-8fbe-4397-9dbb-9b2b20218538