### 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
#if defined(WIDEVINE_CDM_AVAILABLE)
  return key_systems_provider_.IsKeySystemsUpdateNeeded();
#else
  return false;
#endif
}
void RendererClientBase::DidSetUserAgent(const std::string& user_agent) {
#if BUILDFLAG(ENABLE_PRINTING)
  printing::SetAgent(user_agent);
#endif
}
v8::Local<v8::Context> RendererClientBase::GetContext(
    blink::WebLocalFrame* frame,
    v8::Isolate* isolate) const {
  if (isolated_world())
    return frame->WorldScriptContext(isolate, World::ISOLATED_WORLD);
  else
    return frame->MainWorldScriptContext();
}
v8::Local<v8::Value> RendererClientBase::RunScript(
    v8::Local<v8::Context> context,
    v8::Local<v8::String> source) {
  auto maybe_script = v8::Script::Compile(context, source);
  v8::Local<v8::Script> script;
  if (!maybe_script.ToLocal(&script))
    return v8::Local<v8::Value>();
  return script->Run(context).ToLocalChecked();
}
}