In the {WasmEngine::RemoveIsolate} method, we might remove the last
outstanding isolate for the current GC. In that case, finish the GC at
that point.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I7d710756d29265e0552fcc318435133f7db03dda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598706Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61298}