Fix shared function info code replacement.

This fixes a corner case when the unoptimized code for a shared function
info is replaced while the function is enqueued as a flushing candidate.
Since the link field is stored within the code object, the candidates
list got destroyed.

R=hpayer@chromium.org
BUG=v8:169209
TEST=cctest/test-heap/Regress169209

Review URL: https://codereview.chromium.org/11818052

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13361 ce2b1a6d-e550-0410-aec6-3dcde31c8c00