Swap const product = "Gecko" for readonly IDL attribute product

This is a partial cherry-pick from master..

https://github.com/w3c/html/commit/b6126fe342658402fa9c7e1dc3d8b7878d12d210
[giow] (3) Spec navigator.appCodeName (and respec navigator.product) …
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8038
340c8d12-0b0e-0410-8428-c7bf67bfef74
(cherry picked from commit 130d0fe)