### 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
if (!CheckRenderFrame())
    return -1;
  return render_frame_->GetRoutingID();
}
GURL WebFrameMain::URL() const {
  if (!CheckRenderFrame())
    return GURL::EmptyGURL();
  return render_frame_->GetLastCommittedURL();
}
content::RenderFrameHost* WebFrameMain::Top() const {
  if (!CheckRenderFrame())
    return nullptr;
  return render_frame_->GetMainFrame();
}
content::RenderFrameHost* WebFrameMain::Parent() const {
  if (!CheckRenderFrame())
    return nullptr;
  return render_frame_->GetParent();
}
std::vector<content::RenderFrameHost*> WebFrameMain::Frames() const {
  std::vector<content::RenderFrameHost*> frame_hosts;
  if (!CheckRenderFrame())
    return frame_hosts;
  for (auto* rfh : render_frame_->GetFramesInSubtree()) {
    if (rfh->GetParent() == render_frame_)
      frame_hosts.push_back(rfh);
  }
  return frame_hosts;
}
std::vector<content::RenderFrameHost*> WebFrameMain::FramesInSubtree() const {