Pending phantom handle callbacks are not reliably executed if the heap
shuts down. This can cause to memory leaks or other unwanted behaviour,
like in wasm where the NativeModules (held in Managed objects
implemented via phantom handles) unregister from the WasmEngine in the
second-pass callback. This must be executed before tearing down the
WasmEngine.

This CL fixes this by running pending callback synchronously on heap
tear down.

R=ulan@chromium.org, mlippautz@chromium.org

Bug: v8:8208
Change-Id: I27b630c4d8f1fb12309040ea2179b64eed38710a
Reviewed-on: https://chromium-review.googlesource.com/1249101
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56286}