Whitelist the <wbr> element.

This was whitelisted by the PHP parser in change
If4a4882155888606c960237be0fbe7933a85c5f9.

The allowed attributes match those allowed for the <br> element, with
the exception of 'clear'.

Change-Id: I1e41a51487b12ff8c193a91ca2924c2ab1724035
Merge "Whitelist the <wbr> element."