Remove use of Isolate::Current in regexp macro assemblers.

BUG=
R=hpayer@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16026020

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14994 ce2b1a6d-e550-0410-aec6-3dcde31c8c00