This is needed so that we can run Parser on a non-main thread (independent
of the Isolate and the V8 heap).

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/314603004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21841 ce2b1a6d-e550-0410-aec6-3dcde31c8c00