The Isolate* member of ParserBase::FunctionState was only used by
Parser. Removing it makes it clear that there are no isolates in
PreParser. (There's also no Zone, since PreParserTraits::Type::Zone is void.)

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/216883003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20331 ce2b1a6d-e550-0410-aec6-3dcde31c8c00