This makes Parser a bit more independent of Isolate during the recursive descent
phase. That is necessary for making it possible to run Parser on a non-main
thread in the future.

Proof of concept CL for the the "Parser independent of Isolate" work is here:
https://codereview.chromium.org/231073002/

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/289373005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21385 ce2b1a6d-e550-0410-aec6-3dcde31c8c00