Disable code flushing for native SharedFunctionInfo.

R=hpayer@google.com
TEST=cctest/test-api/Threading4

Review URL: https://codereview.chromium.org/15018012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14577 ce2b1a6d-e550-0410-aec6-3dcde31c8c00