### 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_render_frame_observer.cc
blink::WebVector<blink::WebDraggableRegion> webregions =
      render_frame_->GetWebFrame()->GetDocument().DraggableRegions();
  std::vector<mojom::DraggableRegionPtr> regions;
  for (auto& webregion : webregions) {
    auto region = mojom::DraggableRegion::New();
    render_frame_->GetRenderView()->ConvertViewportToWindowViaWidget(
        &webregion.bounds);
    region->bounds = webregion.bounds;
    region->draggable = webregion.draggable;
    regions.push_back(std::move(region));
  }
  mojom::ElectronBrowserPtr browser_ptr;
  render_frame_->GetRemoteInterfaces()->GetInterface(
      mojo::MakeRequest(&browser_ptr));
  browser_ptr->UpdateDraggableRegions(std::move(regions));
}
void AtomRenderFrameObserver::WillReleaseScriptContext(