[giow] (2) Define setting valueAsNumber to Infinity or NaN.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=20496
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@7723 340c8d12-0b0e-0410-8428-c7bf67bfef74