This avoids the need to clear optimized code maps within the GC by just
preventing entries being added in the first place, whenever a snapshot
is being constructed. The main goal here is to simplify the logic in the
already complex visitor for our shared function info objects.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1411513005

Cr-Commit-Position: refs/heads/master@{#31852}