The checks were at the wrong abstraction level, JSObject is the right place for
this check. Note that other uses of ValueAtPut either don't need a check at all
(like the one used for copying boilerplate) or do the check for themselves.

Review URL: https://chromiumcodereview.appspot.com/9417044

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10741 ce2b1a6d-e550-0410-aec6-3dcde31c8c00