Enable incremental marking when --expose-gc is turned on.

BUG=
R=mstarzinger@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/139343007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18691 ce2b1a6d-e550-0410-aec6-3dcde31c8c00