Factory::CopyCode was using ProcessBlackAllocatedObject and
WriteBarrierForCode(Code) to handle write barriers for that newly
created code object. But even when used in tandem with each other they
would miss OLD_TO_NEW references in the code object header.

This CL simplifies Factory::CopyCode by letting
WriteBarrierForCode(Code) handle all outgoing pointers of that code
object (not just a subset of RelocInfos) by implementing an
ObjectVisitor. This removes the need for ProcessBlackAllocatedObject.

Since Factory::CopyCode was the only user of
ProcessBlackAllocatedObject, we can also remove all the object
revisiting logic in the main thread marker.

Bug: v8:11708
Change-Id: I7d9b12eb0a76ba41a38efc147f44556ddc941a96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3810186Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82212}