Note that we still need to migrate from sometimes emitting those
comments by hand to passing a reason explicitly, but this can be done
incrementally in separate CLs.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/582743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24061 ce2b1a6d-e550-0410-aec6-3dcde31c8c00