This splits up js-heap-broker.cc into

- js-heap-broker.cc: the JSHeapBroker impl, and
- heap-refs.cc: ObjectRef and ObjectData impls, as well as two
  JSHeapBroker functions that closely deal with refs/data objects.

Bug: v8:7790
Change-Id: I7e097b60cdec4fd61b39d7de9752d536ac313cbe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2833919Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74057}