### 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.h
static v8::Local<v8::Value> RunScript(v8::Local<v8::Context> context,
                                        v8::Local<v8::String> source);
 protected:
  void AddRenderBindings(v8::Isolate* isolate,
                         v8::Local<v8::Object> binding_object);
  
  void RenderThreadStarted() override;
  void RenderFrameCreated(content::RenderFrame*) override;
  std::unique_ptr<blink::WebSpeechSynthesizer> OverrideSpeechSynthesizer(
      blink::WebSpeechSynthesizerClient* client) override;
  bool OverrideCreatePlugin(content::RenderFrame* render_frame,
                            const blink::WebPluginParams& params,
                            blink::WebPlugin** plugin) override;
  void AddSupportedKeySystems(
      std::vector<std::unique_ptr<::media::KeySystemProperties>>* key_systems)
      override;
  bool IsKeySystemsUpdateNeeded() override;
  void DidSetUserAgent(const std::string& user_agent) override;
 private:
#if defined(WIDEVINE_CDM_AVAILABLE)
  ChromeKeySystemsProvider key_systems_provider_;
#endif
  bool isolated_world_;
  std::string renderer_client_id_;
  
  int64_t next_context_id_ = 0;
};
}  
#endif