Move fixing up Filler objects after deserialization from the
StartupDeserializer to the ReadOnlyDeserializer since that's what
deserializes the Filler maps.

Since only RO_SPACE can contain such objects, skip iterating over all
the spaces and just fix them up directly in RO_SPACE. To this end, the
PagedSpace code is moved to ReadOnlySpace and the Heap fix up method is
removed.

Change-Id: I7a01f1ef298e6d5e74d3173620fb7764c3b598f2
Reviewed-on: https://chromium-review.googlesource.com/c/1299013Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57015}