### Issue Details

* **Electron Version:**
  * 13.0.0-nightly.20201203
* **Operating System:**
  * macOS
* **Last Known Working Electron version:**
  * N/A

### Expected Behavior
`WebFrameMain.frames` should never contain undefined

### Actual Behavior
```
Uncaught Exception:
TypeError: Cannot read property 'processId' of undefined
    at id (/private/var/folders/_7/jm5cr84j7jd4v7btf2d3bqwm0000gn/T/tmp-59557-p4DTr3KWOsNk/main.js:4:38)
    at pft (/private/var/folders/_7/jm5cr84j7jd4v7btf2d3bqwm0000gn/T/tmp-59557-p4DTr3KWOsNk/main.js:6:36)
    at Timeout._onTimeout (/private/var/folders/_7/jm5cr84j7jd4v7btf2d3bqwm0000gn/T/tmp-59557-p4DTr3KWOsNk/main.js:27:5)
    at listOnTimeout (internal/timers.js:554:17)
    at processTimers (internal/timers.js:497:7)
```

### To Reproduce
Run https://gist.github.com/f46823470955034b4a456b6275862bfb for a short while and you'll observe the above error.

cc @samuelmaddock 

shell/browser/api/electron_api_web_frame_main.cc
const std::string& channel,
                               v8::Local<v8::Value> message_value,
                               base::Optional<v8::Local<v8::Value>> transfer) {
  blink::TransferableMessage transferable_message;
  if (!electron::SerializeV8Value(isolate, message_value,
                                  &transferable_message)) {
    
    return;
  }
  std::vector<gin::Handle<MessagePort>> wrapped_ports;
  if (transfer) {
    if (!gin::ConvertFromV8(isolate, *transfer, &wrapped_ports)) {
      isolate->ThrowException(v8::Exception::Error(
          gin::StringToV8(isolate, "Invalid value for transfer")));
      return;
    }
  }
  bool threw_exception = false;
  transferable_message.ports =
      MessagePort::DisentanglePorts(isolate, wrapped_ports, &threw_exception);
  if (threw_exception)
    return;
  if (!CheckRenderFrame())