Between determining the set of wasm code objects to free, and actually
freeing them, we should not give up the mutex of the wasm engine.
Otherwise, a NativeModule can die in-between, and we would access a
stale pointer.
This fixes some flakes seen on the TSan bots with --stress-wasm-code-gc.

R=mstarzinger@chromium.org

Bug: v8:8217, v8:9200
Change-Id: Iad5b47379b5be6269180094cfeb2a2f2dfefb425
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1593340Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61204}