* Electron version: 1.4.6
* Operating system: Linux Mint 18

### Expected behavior

Printing to PDF works even when using a renderer sandbox

### Actual behavior

The callback passed to printToPDF never gets called, so my app hangs. The sample code logs "Printing" on the console, but never "Success." Changing the sandbox flag to false fixes the issue.

### How to reproduce

```
const {BrowserWindow, app} = require('electron')

app.on('ready', function() {
  var webPreferences = {
    sandbox: true
  }

  var mainWindow = new BrowserWindow({
    webPreferences: webPreferences
  })

  mainWindow.webContents.on('did-finish-load', function() {
    console.log('Printing')
    mainWindow.webContents.printToPDF({}, function(error, data) {
      if (error) {
        console.error('Failed to print')
      } else {
        console.log('Success')
      }
    })
  })

  mainWindow.loadURL('https://www.google.com')
})
```
atom/renderer/atom_sandboxed_renderer_client.cc
public:
  AtomSandboxedRenderFrameObserver(content::RenderFrame* frame,
                                   AtomSandboxedRendererClient* renderer_client)
      : content::RenderFrameObserver(frame),
        render_frame_(frame),
        world_id_(-1),
        renderer_client_(renderer_client) {}
  
  void DidClearWindowObject() override {