this cl also moves all accesses to the default isolate behind EnsureDefaultIsolate

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/24999002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17049 ce2b1a6d-e550-0410-aec6-3dcde31c8c00