Separate JSON parsing from the JavaScript parser.

Separate JSON parsing from the JavaScript parser.
Switch JSON parsing to creating the value directly instead of createing
code to create the value.

Review URL: http://codereview.chromium.org/4135004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00