This flag will be passed to GC prologue/epilogue callbacks if GC was forced through GC extension.

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

Review URL: https://codereview.chromium.org/104023011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18558 ce2b1a6d-e550-0410-aec6-3dcde31c8c00