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}