This is a reland of a5fa211f

des_checksum and call_once_run were undefined and unused respectively when
shared read-only heap was enabled. Fixed with a copious amounts of USE.

Original change's description:
> [roheap] Check that ro-heap is always passed the same read-only snapshot
>
> Previously the ReadOnlyHeap simply discarded all but the first
> ReadOnlyDeseralizer. ClearSharedHeapForTest should be called if using a
> new ReadOnlyDeserializer (this might change in the future).
>
> Remove an obsolete 'StartupSerializerRootMapDependencies' test. It used
> to test Map::WeakCellForMap which doesn't exist anymore and was
> difficult to adapt to a shared read-only heap.
>
> Bug: v8:7464
> Change-Id: I64b8e953b0e3466e003541ec8a9321e439a01d33
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1660612
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Dan Elphick <delphick@chromium.org>
> Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
> Cr-Commit-Position: refs/heads/master@{#62250}

TBR: yangguo@chromium.org
Bug: v8:7464
Change-Id: Id66e781be890c5ed03d066f8c62de703d5cb435e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667415Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62302}