Valgrind fix: always initialize HCheckTableEntry->is_stable_

BUG=chromium:343621
LOG=N
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/166413002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19371 ce2b1a6d-e550-0410-aec6-3dcde31c8c00