Prevent flushing of code that was set with %SetCode.

This makes sure that shared function infos that break the one-to-one
mapping to code are marked as un-flushable. Otherwise enqueuing through
the GC meta-data field in the code object doesn't work.

R=rossberg@chromium.org
TEST=cctest/test-api/Threading4

Review URL: https://codereview.chromium.org/14710015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14635 ce2b1a6d-e550-0410-aec6-3dcde31c8c00