NormalPageMemoryRegion is a span of 10 pages, all of which must belong
to the same space. This requirement imposes a fragmentation issue for virtual space, which is not ideal for the current 2GB cage
configuration.

The CL fixes this by mixing pages of different spaces inside the same
NormalPageMemoryRegion. With cage it's actually not necessary anymore
to have NormalPageMemoryRegion, but we keep it to allow the code to be
uniform for cage/non-cage configurations.

There is no type confusion across spaces, since pages (even empty) are
never shared between spaces. In addition, the shared cage puts an
additional memory constraint on the GC. So, there is no security benefit
in having NormalPageMemoryRegion assigned to a single space.

Savings in reserved address space:
cnn:2021: 14%
facebook_infinite_scroll:2018: 23%

Bug: chromium:1325007, chromium:1352649
Change-Id: I7b49032d581dd56feb8633734a1f37803e9526c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3840749Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82584}