This CL adds to the existing experimental implementation of the
object start bitmap, that is evaluated as a mechanism for resolving
inner pointers (behind the flag v8_enable_conservative_stack_scanning).

It fixes method ObjectStartBitmap::FindBasePtr to ensure that the
correct base pointer is returned, even if the bitmap is not fully
populated (e.g., with object evacuation or inline object allocation).
This method now recalculates the part of the bitmap that is
required for returning the correct result, by iterating through
objects of the page. A special constructor has been introduced to the
PagedSpaceObjectIterator for this purpose.

It also moves the existing inline methods of ObjectStartBitmap to a
new -inl.h header file, to avoid circular dependencies.

Bug: v8:12851
Change-Id: Iabd0df020bee3bb63ef9d4888591b25d24d79dd9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3641179Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80538}