Introduce an API mirroring the gc extension

BUG=none
R=mstarzinger@chromium.org, svenpanne@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/131443008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18563 ce2b1a6d-e550-0410-aec6-3dcde31c8c00