Handle boxed length in JSON stringify.

Here is the test that used to fail in Smi::cast:

JSON.stringify(new Array(4000000000));

I am not adding the test because it takes huge amount of time before it
runs out of memory.

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/247063003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20898 ce2b1a6d-e550-0410-aec6-3dcde31c8c00