The CL is a prerequisite for the shared cage. Instead of storing
state variables (is_incremental_marking_in_progress,
is_young_generation_enabled) in the cage metadata, the CL moves them to
HeapHandle. The HeapHandle pointer is now retrieved from page-headers.

To make sure that the write-barrier code is better optimized, the
HeapHandle definition is moved to internal/ headers. The part of
BasePage that contains HeapBase (i.e. HeapHandle) pointer is also
extracted and moved to the headers.

Bug: v8:12231
Change-Id: I44bf65d99a621d9548e4250386cf87476ca186ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3689730
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81005}