### 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
g_render_frame_map.Get().emplace(rfh, this);
}
WebFrameMain::~WebFrameMain() {
  MarkRenderFrameDisposed();
}
void WebFrameMain::MarkRenderFrameDisposed() {
  g_render_frame_map.Get().erase(render_frame_);
  render_frame_disposed_ = true;
}
bool WebFrameMain::CheckRenderFrame() const {
  if (render_frame_disposed_) {
    v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
    v8::Locker locker(isolate);
    v8::HandleScope scope(isolate);
    gin_helper::ErrorThrower(isolate).ThrowError(
        "Render frame was disposed before WebFrameMain could be accessed");
    return false;
  }
  return true;
}
v8::Local<v8::Promise> WebFrameMain::ExecuteJavaScript(
    gin::Arguments* args,
    const base::string16& code) {
  gin_helper::Promise<base::Value> promise(args->isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();