instantiate default isolate on v8::Isolate::GetCurrent()

TBR=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/25611003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17060 ce2b1a6d-e550-0410-aec6-3dcde31c8c00