* Electron version: 1.7.1
* Operating system: MacOS

### Expected behavior

The popup should have `window.opener` set

### Actual behavior

I was using native `window.open` in sandbox mode until this PR was merged : https://github.com/electron/electron/pull/8963

I switched to non-sandbox mode using the version 1.7.1 and what happen is that, if your main app is embedded in an iframe and if it open a new window using `window.open`, the new window won't have `window.opener` set

If I load the same app directly in the main window and not using the iframe, it is working as expected

I am using the code from https://github.com/electron/electron/blob/master/docs/api/window-open.md

### How to reproduce

In the main window load in an iframe your app.

In the app open a popup with `window.open`

The new popup does not have `window.opener` set
atom/browser/atom_browser_client.cc
return false;
    } else {
      *no_javascript_access = false;
      return true;
    }
  }
  if (delegate_) {
    content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
        base::Bind(&api::App::OnCreateWindow,
                   base::Unretained(static_cast<api::App*>(delegate_)),
                                    target_url,
                                    frame_name,
                                    disposition,
                                    additional_features,
                                    body,
                                    opener_render_process_id,
                                    opener_render_frame_id));
  }
  return false;
}
void AtomBrowserClient::GetAdditionalAllowedSchemesForFileSystem(
    std::vector<std::string>* additional_schemes) {
  auto schemes_list = api::GetStandardSchemes();
  if (!schemes_list.empty())
    additional_schemes->insert(additional_schemes->end(),
                               schemes_list.begin(),
                               schemes_list.end());
  additional_schemes->push_back(content::kChromeDevToolsScheme);
}
brightray::BrowserMainParts* AtomBrowserClient::OverrideCreateBrowserMainParts(