Moved the storage of the last script id into the heap to make it part of the serialized data when starting V8 on a snapshot.

Currently the script ids wrap when positive smi value is exhausted.
Review URL: http://codereview.chromium.org/43008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1474 ce2b1a6d-e550-0410-aec6-3dcde31c8c00