Use stored Isolate pointer instead of Isolate::Current()

Fixed a couple of places where stored pointer to the isolate can be used instead of reading from thread local storage.

BUG=None
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/18418003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15457 ce2b1a6d-e550-0410-aec6-3dcde31c8c00