This makes sure that the GC fully flushes the optimized code map when
the serializer is preparing a snapshot. Otherwise closures and contexts
could end up in the startup snapshot.

R=hpayer@chromium.org
TEST=cctest/test-serialize/SerializeInternalReference

Review URL: https://codereview.chromium.org/1215063007

Cr-Commit-Position: refs/heads/master@{#29461}