### 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/renderer_client_base.cc
const char* switch_name) {
  std::string custom_schemes = command_line->GetSwitchValueASCII(switch_name);
  return base::SplitString(custom_schemes, ",", base::TRIM_WHITESPACE,
                           base::SPLIT_WANT_NONEMPTY);
}
}  
RendererClientBase::RendererClientBase() {
  auto* command_line = base::CommandLine::ForCurrentProcess();
  
  std::vector<std::string> standard_schemes_list =
      ParseSchemesCLISwitch(command_line, switches::kStandardSchemes);
  for (const std::string& scheme : standard_schemes_list)
    url::AddStandardScheme(scheme.c_str(), url::SCHEME_WITH_HOST);
  isolated_world_ = base::CommandLine::ForCurrentProcess()->HasSwitch(
      switches::kContextIsolation);
  
  
  
  
  DCHECK(command_line->HasSwitch(::switches::kRendererClientId));
  renderer_client_id_ =
      command_line->GetSwitchValueASCII(::switches::kRendererClientId);
}
RendererClientBase::~RendererClientBase() {}