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/+/1660612Reviewed-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}