When Heap::TearDown is called, parts of the Isolate are already gone
(specifically: Managed<> objects, which includes Wasm NativeModules).
Since heap verification can depend on these parts (e.g. to find Code
objects belonging to current activations on the stack), we should do
it before tearing down things. Heap::StartTearDown is a suitable way
to achieve that.

Bug: v8:9209
Change-Id: I44094b19e16a4f372eb14ab363d8b4a65182f38a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1993968
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65684}