Refactored the code for handling debug step in in the runtime system into one function. For constructors this also means that step in will no longer step into the code for the builtins context.
Review URL: http://codereview.chromium.org/15035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00