This is a reland of 902f48bd, fixed
to avoid lock inversion problems detected by TSan.

Original change's description:
> [wasm][debug] Fix tier down for multiple isolates
>
> If multiple isolates are using the same module, we need to keep it
> tiered down as long as any isolate still has a debugger open.
> Also, we cannot short-cut the {NativeModule::TierDown} method, since the
> previously triggered tier down might not have finished yet.
> For now, each isolate starts an independent tier down (i.e. a full
> recompilation). We could optimize this later by skipping functions that
> are already tiered down, or are already scheduled for tier down, but we
> still need to wait for tier-down to finish on each isolate.
>
> R=thibaudm@chromium.org
>
> Bug: v8:10359
> Change-Id: I7ea6a6f5d3977e48718ac5bc94f9831541f6173f
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190758
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67716}

Bug: v8:10359
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: Ie98cf073fc79e5c6991df6d4466de7b560274070
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2194451
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67754}