Serializer: reorder root list items.

Serializer: reorder root list items.

The first 32 root list items can be encoded very efficiently.
By reordering, we can better encode some often referenced root objects.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1019243002

Cr-Commit-Position: refs/heads/master@{#27293}