Cleanup of Isolate::Current() and FACTORY macro usage in execution.cc.

BUG=none
TEST=none

Patch from Peter Varga <pvarga@inf.u-szeged.hu>.

Review URL: http://codereview.chromium.org/6759029

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7464 ce2b1a6d-e550-0410-aec6-3dcde31c8c00