Fix %SetFlags("--stress-compaction")

BUG=369943
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/261253006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21260 ce2b1a6d-e550-0410-aec6-3dcde31c8c00