Implement non-zero dimension attribute parsing

Fixes #8445

The only attributes I found that we have implemented that uses non-zero
dimenion attributes:

* `width` for `<td>` and `<th>` (table cells)
* `width` for `<table>`

I updated these implementations to use the new non-zero dimension
attribute parsing and added associated regression tests.

components/script/dom/webidls/HTMLTableCellElement.webidl

@@ -19,7 +19,7 @@ partial interface HTMLTableCellElement {
   //         attribute DOMString align;
   //         attribute DOMString axis;
   //         attribute DOMString height;
-  //         attribute DOMString width;
+  attribute DOMString width;
 
   //         attribute DOMString ch;
   //         attribute DOMString chOff;