This is a reland of 4d7ad46d

Original change's description:
> Introduce CodeReference
>
> Add a struct CodeReference that can be stack allocated to pass a
> reference to either an on-heap code object or off-heap WasmCode object
> in a gc safe manner. The struct also provides a common interface such
> that code can be written independently of the kind of code object it
> references.
>
> Change-Id: I5a6f74462e6e141d167c7fd9bac8c21941fd83b1
> Reviewed-on: https://chromium-review.googlesource.com/977905
> Commit-Queue: Stephan Herhut <herhut@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52580}

Change-Id: I40861474fe4a3efd72e6c59e2e7b847ab6772735
Reviewed-on: https://chromium-review.googlesource.com/1013939
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52665}