Use RegisterSize to count the pushq size and the argument slot size from JS to C++

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/213413010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20317 ce2b1a6d-e550-0410-aec6-3dcde31c8c00