This does not include moving function definitions from objects-inl.h
to heap-object-inl.h, because that would be messy for no immediate
benefit.
This is in preparation for merging HeapObjectPtr into HeapObject.

Bug: v8:5402
Change-Id: Id170d9cac8ebabb2876b85bad29ce90fe02d842d
Reviewed-on: https://chromium-review.googlesource.com/c/1386491Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58401}