Revert "Unify calling to GenerateFastApiCallBody before stubbing it"

This reverts commit r18714 for breaking webkit tests with an assert.

TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/144143002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18720 ce2b1a6d-e550-0410-aec6-3dcde31c8c00