### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  5.0.1
* **Operating System:**
  windows
* **Last Known Working Electron version:**
  n/a

### Expected Behavior
A preload script should run only 1 time for each webContents / iframe

### Actual Behavior
The preload script appears to run multiple times

### To Reproduce
https://github.com/Tim91084/electron-node-subframe-issue/tree/node-in-subframe-runs-too-often

```sh
$ git clone https://github.com/Tim91084/electron-node-subframe-issue.git -b node-in-subframe-runs-too-often
$ npm install
$ npm start || electron .
```

### Screenshots
![image](https://user-images.githubusercontent.com/5086428/58273769-63b9d900-7d5f-11e9-8d30-1525dea34ce3.png)

shell/renderer/atom_renderer_client.cc
const GURL& url,
                                    const std::string& http_method,
                                    bool is_initial_navigation,
                                    bool is_server_redirect) {
  
  
  
  
  return http_method == "GET";
}
void AtomRendererClient::DidInitializeWorkerContextOnWorkerThread(
    v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextCreated(context);
  }
}
void AtomRendererClient::WillDestroyWorkerContextOnWorkerThread(