Fix memory leak caused by treating Code::next_code_link as strong in marker.

BUG=
TEST=test/cctest/NextCodeLinkIsWeak
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/181833004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19897 ce2b1a6d-e550-0410-aec6-3dcde31c8c00