Instead allocating the bitmap with malloc, we now reserve a block
at the start of the memory chunk. This CL is a partial revert of
https://chromium-review.googlesource.com/c/v8/v8/+/1254125
Additionally it refactors field offset computation and moves them
to MemoryChunkLayout.

Having the bitmap in the memory chunk simplifies sharing of RO pages
and also solves the malloc fragmentation issues.

Bug: chromium:1073140
Change-Id: Ibc04f48921fc9496370858ce4c25c56b31c93c89
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2289979
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68783}