Let Heap::MakeHeapIterable() and Heap::FreeLinearAllocationAreas() also
handle the new space to be more uniform between spaces. Also removes
Heap::EnsureFillerObjectAtTop() in favor of
NewSpace::MakeLinearAllocationAreaIterable().

Bug: v8:10315
Change-Id: I7d28c1e95e433c4bc5a4a1a1f3aa8d71c43b8887
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3281926
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77905}