If we do not clear next links during serialization, the
serializer would simply follow those links and serialize
arbitrary objects held by weak cells. This breaks the
invariant in the code serializer, which crashes if it
sees context-dependent objects.

R=ulan@chromium.org
BUG=chromium:503552
LOG=Y

Review URL: https://codereview.chromium.org/1203973002

Cr-Commit-Position: refs/heads/master@{#29255}