[giow] (3) Make DOMStringMap use [OverrideBuiltins]
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=19611
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@7956 340c8d12-0b0e-0410-8428-c7bf67bfef74

 Author:    ianh <ianh@340c8d12-0b0e-0410-8428-c7bf67bfef74>