None of the values can in fact be negative, while object IDs
can have the highest bit set which lead to negative values appear
in the snapshot that front-end is unable to parse.

Review URL: https://chromiumcodereview.appspot.com/10356143

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11553 ce2b1a6d-e550-0410-aec6-3dcde31c8c00