Correctly set the length of string before creating filler object in the json parser (fixes crbug 84186).

Testcase created based on the supplied test case from the bug report, but using json parse directly instead of through the chrome javascript console. 
Review URL: http://codereview.chromium.org/7084023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8089 ce2b1a6d-e550-0410-aec6-3dcde31c8c00