### 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
#include "shell/renderer/renderer_client_base.h"
#include <memory>
#include <string>
#include <vector>
#include "base/command_line.h"
#include "base/strings/string_split.h"
#include "base/strings/stringprintf.h"
#include "content/common/buildflags.h"
#include "content/public/common/content_constants.h"
#include "content/public/common/content_switches.h"
#include "content/public/renderer/render_frame.h"
#include "content/public/renderer/render_view.h"
#include "electron/buildflags/buildflags.h"
#include "native_mate/dictionary.h"
#include "printing/buildflags/buildflags.h"
#include "shell/common/color_util.h"
#include "shell/common/native_mate_converters/value_converter.h"
#include "shell/common/options_switches.h"
#include "shell/renderer/atom_autofill_agent.h"
#include "shell/renderer/atom_render_frame_observer.h"
#include "shell/renderer/content_settings_observer.h"
#include "shell/renderer/electron_api_service_impl.h"
#include "third_party/blink/public/common/associated_interfaces/associated_interface_registry.h"
#include "third_party/blink/public/web/blink.h"