The split is rudimental and now is not needed at all:
- as part of the shared-cage effort we added HeapHandle pointer to the
  BasePageHandle class (on the API side);
- for the value-full barrier we get HeapHandle from bitmasking the
  value;
- for the value-less barrier we get it from the callback provided by the
  caller.

The CL entirely removes the split and uses the single
BoundedPageAllocator. A minor note: the conservative stack scanning can
become sligthly more expensive.

Bug: chromium:1361582, chromium:1325007
Change-Id: I2a8aded3dd12037998f36341c68af8e23b0dcd88
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899320Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83232}