Sometimes we were triggering a wasm code GC at a time where all native
modules just died. Thus, no isolates took part in that GC, and it never
finished (because no isolate would ever call {ReportLiveCodeForGC}).
This never-ending GC would then block all other GCs for the rest of the
life of this wasm engine.

This CL fixes this by just finishing the GC immediately if no isolates
are outstanding.

R=ahaas@chromium.org

Change-Id: I4c25dd6ba4132cf9f72de39c30da5d5cba0526ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2150588
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67154}