v8: fix tracking of longest_step in IncrementalMarking.

longest_step is only output at the end of a mark-compact. We shouldn't
reset it after a Scavenge.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/395153003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22432 ce2b1a6d-e550-0410-aec6-3dcde31c8c00