This CL implements stack scanning for cppgc.
Given a value on the stack, the MarkingVisitor uses
PageBackend::Lookup to checks whether that address is on
the heap. If it is, BasePage::TryObjectHeaderFromInnerAddress
(introduced in this CL) is used to get the relevant object
header. Note that random addresses on the heap might point to
free memory, object-start-bitmap, etc.

If a valid object header is found:
* If the object is not in construction, the GCInfoIndex is used
the get the relevant Trace method and the object is traced.
* Otherwise, the object is conservatively scanned - i.e. the
payload of the object is iterated word by word and each word is
treated as a possible pointer.

Only addresses pointing to the payload on non-free objects are
traced.

BasePage::TryObjectHeaderFromInnerAddress assumes on LAB on the
relevant space, thus all LABs are reset before scanning the stack.

Bug: chromium:1056170
Change-Id: I172850f6f1bbb6f0efca8e44ad8fdfe222977b9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190426
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67795}