Support stepping into generator function.

R=aandrey@chromium.org, wingo@igalia.com
BUG=v8:3572
LOG=Y

Review URL: https://codereview.chromium.org/544953005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24000 ce2b1a6d-e550-0410-aec6-3dcde31c8c00