### 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
render_frame->GetAssociatedInterfaceRegistry()->AddInterface(
      base::BindRepeating(&ElectronApiServiceImpl::CreateMojoService,
                          render_frame, this));
#if BUILDFLAG(ENABLE_PDF_VIEWER)
  
  blink::WebSecurityPolicy::AddOriginAccessWhitelistEntry(
      GURL(kPdfViewerUIOrigin), "file", "", true);
#endif  
  content::RenderView* render_view = render_frame->GetRenderView();
  if (render_frame->IsMainFrame() && render_view) {
    blink::WebView* webview = render_view->GetWebView();
    if (webview) {
      base::CommandLine* cmd = base::CommandLine::ForCurrentProcess();
      if (cmd->HasSwitch(switches::kGuestInstanceID)) {  
        webview->SetBaseBackgroundColor(SK_ColorTRANSPARENT);
      } else {  
        std::string name = cmd->GetSwitchValueASCII(switches::kBackgroundColor);
        SkColor color =
            name.empty() ? SK_ColorTRANSPARENT : ParseHexColor(name);
        webview->SetBaseBackgroundColor(color);
      }
    }
  }
}
void RendererClientBase::DidClearWindowObject(
    content::RenderFrame* render_frame) {
  
  render_frame->GetWebFrame()->ExecuteScript(blink::WebScriptSource("void 0"));
}
std::unique_ptr<blink::WebSpeechSynthesizer>