This is a reland of 0938188f.
The test is skipped for now, until we figure out a way to make wasm
code GC deterministic.

Original change's description:
> [wasm][debug] Garbage-collect stepping code
>
> All wasm code has an initial ref count of 1, in the expectation that it
> will be added to the code table. When the code is removed from that
> table, the ref count will be decremented.
> Stepping code (and also other code under special circumstances) will not
> be added to the code table though. Hence the ref count will never be
> decremented below 1, and the code will never be garbage-collected.
>
> This CL fixes this, by decrementing the ref count if the code is not
> added to the code table.
> Note that the code will only be collected if no isolate is currently
> using it, so it won't be collected while still in use for stepping.
>
> R=thibaudm@chromium.org
>
> Bug: chromium:1168564
> Change-Id: I3047753591cbc52689ca019e9548ec58c237b835
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2649040
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72354}

Bug: chromium:1168564
Cq-Include-Trybots: luci.v8.try:v8_linux64_asan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_mac64_asan_rel_ng
Change-Id: Idb3baec713e0732e51e13b665ac6ac86fdfec969
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2652488Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72365}