Use 0 instead of undefined for uninitialized stub key.

The CPU profiler looks at uninitialized code objects, which triggers an assertion.

TBR=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/404113003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22507 ce2b1a6d-e550-0410-aec6-3dcde31c8c00