When serializing/deserializing RO_SPACE objects in CodeSerializer use
BackReferences to avoid re-allocating inside RO_SPACE.

Change-Id: Ie9063a43a4f758f0401ad59dfcc61c4b759591bc
Reviewed-on: https://chromium-review.googlesource.com/997837
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52429}