Previously, in order to get immortal immovable objects onto
the first page, the serializer would iterate the root list
twice. The first time it would prioritize immortal immovables.
The second time it would serialize the rest.

This does not guarantee that immortal immovable objects
actually end up on the first page, and by now this is not
necessary anymore, since we mark all pages created during
heap init as immortal immovable pages.

R=mlippautz@chromium.org

Change-Id: Ie95fcd779377a75337621ba862bc1a745ed5cbaa
Reviewed-on: https://chromium-review.googlesource.com/768731
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49468}