### 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.h
#ifndef SHELL_BROWSER_API_ELECTRON_API_WEB_FRAME_MAIN_H_
#define SHELL_BROWSER_API_ELECTRON_API_WEB_FRAME_MAIN_H_
#include <memory>
#include <string>
#include <vector>
#include "base/process/process.h"
#include "gin/handle.h"
#include "gin/wrappable.h"
#include "shell/common/gin_helper/constructible.h"
class GURL;
namespace content {
class RenderFrameHost;
}
namespace gin {
class Arguments;
}
namespace gin_helper {
class Dictionary;
}
namespace electron {
namespace api {
class WebFrameMain : public gin::Wrappable<WebFrameMain>,
                     public gin_helper::Constructible<WebFrameMain> {
 public:
  
  static gin::Handle<WebFrameMain> New(v8::Isolate* isolate);