During graph scheduling, traverse the roots in one go.

This makes the scheduler more than 30x faster on zlib.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/474963002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23137 ce2b1a6d-e550-0410-aec6-3dcde31c8c00