introduced in https://crrev.com/72f884a19fa4434bba6fc0e013ec4ea0a2366893

The regression comes from adding the next weak field of AllocationSite
as a hidden reference into the snapshot.

Before 72f884 the reference was implicitly ignored because the body
descriptor of AllocationSite did not include it.

This patch explicitly skip the next weak field of AllocationSite.

BUG=chromium:630027

Review-Url: https://codereview.chromium.org/2189643004
Cr-Commit-Position: refs/heads/master@{#38211}