- Initialize the result array with holes if we concat a double array into an object array, since it may cause a marking step while boxing a double.
- Ensure we go holey if we are concatting any holey array.

Review URL: https://chromiumcodereview.appspot.com/11413142

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13038 ce2b1a6d-e550-0410-aec6-3dcde31c8c00