### 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_sandboxed_renderer_client.cc
void AtomSandboxedRendererClient::InitializeBindings(
    v8::Local<v8::Object> binding,
    v8::Local<v8::Context> context,
    bool is_main_frame) {
  auto* isolate = context->GetIsolate();
  mate::Dictionary b(isolate, binding);
  b.SetMethod("get", GetBinding);
  b.SetMethod("createPreloadScript", CreatePreloadScript);
  mate::Dictionary process = mate::Dictionary::CreateEmpty(isolate);
  b.Set("process", process);
  ElectronBindings::BindProcess(isolate, &process, metrics_.get());
  process.Set("argv", base::CommandLine::ForCurrentProcess()->argv());
  process.SetReadOnly("pid", base::GetCurrentProcId());
  process.SetReadOnly("sandboxed", true);
  process.SetReadOnly("type", "renderer");
  process.SetReadOnly("isMainFrame", is_main_frame);
  
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (command_line->HasSwitch(switches::kGuestInstanceID))
    b.Set(options::kGuestInstanceID,
          command_line->GetSwitchValueASCII(switches::kGuestInstanceID));
}
void AtomSandboxedRendererClient::RenderFrameCreated(
    content::RenderFrame* render_frame) {
  new AtomRenderFrameObserver(render_frame, this);
  RendererClientBase::RenderFrameCreated(render_frame);
}
void AtomSandboxedRendererClient::RenderViewCreated(