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
[Auto-roll] Bump up version to 3.29.3.0

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/482523002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23138 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge