JSStack::ensureCapacityFor() will be able to handle both the JS stack on the C stack or a separate stack. Clients should use JSStack::ensureCapacity() when doing stack checks for JS stack needs in place of JSStack::grow(). JSStack::grow() is now private.
Created attachment 219832 [details] the patch.
Landed in r160960 on the jsCStack: <http://trac.webkit.org/r160960>. Still need a review.
Thanks for the review. Review status updated in r161118: <http://trac.webkit.org/r161118>.