Immortal immovable roots must be allocated on the first page of the space.
If serializing the root list exceeds the first page, immortal immovable root
objects might end up outside of the first page. That could cause missing
write barriers.

We now iterate the root list twice. The first time we only serialize immortal
immovable root objects. The second time we serialize the rest.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1811913002

Cr-Commit-Position: refs/heads/master@{#34859}