### 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
RendererClientBase::OverrideSpeechSynthesizer(
    blink::WebSpeechSynthesizerClient* client) {
#if BUILDFLAG(ENABLE_TTS)
  return std::make_unique<TtsDispatcher>(client);
#else
  return nullptr;
#endif
}
bool RendererClientBase::OverrideCreatePlugin(
    content::RenderFrame* render_frame,
    const blink::WebPluginParams& params,
    blink::WebPlugin** plugin) {
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (params.mime_type.Utf8() == content::kBrowserPluginMimeType ||
#if BUILDFLAG(ENABLE_PDF_VIEWER)
      params.mime_type.Utf8() == kPdfPluginMimeType ||
#endif  
      command_line->HasSwitch(switches::kEnablePlugins))
    return false;
  *plugin = nullptr;
  return true;
}
void RendererClientBase::AddSupportedKeySystems(
    std::vector<std::unique_ptr<::media::KeySystemProperties>>* key_systems) {
#if defined(WIDEVINE_CDM_AVAILABLE)
  key_systems_provider_.AddSupportedKeySystems(key_systems);
#endif
}
bool RendererClientBase::IsKeySystemsUpdateNeeded() {