Fix step in positions (include various calls and exclude current pc point), add a test

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/22198002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16100 ce2b1a6d-e550-0410-aec6-3dcde31c8c00