Precisely measure duration of mark and sweep phases. Changed print_cumulative_gc_stat flag to only print GC statistics at end of program.

BUG=

Review URL: https://codereview.chromium.org/11595006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13551 ce2b1a6d-e550-0410-aec6-3dcde31c8c00