* 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
#include "atom/browser/atom_browser_client.h"
#if defined(OS_WIN)
#include <shlobj.h>
#endif
#include "atom/browser/api/atom_api_app.h"
#include "atom/browser/api/atom_api_protocol.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/atom_quota_permission_context.h"
#include "atom/browser/atom_resource_dispatcher_host_delegate.h"
#include "atom/browser/atom_speech_recognition_manager_delegate.h"
#include "atom/browser/child_web_contents_tracker.h"
#include "atom/browser/native_window.h"
#include "atom/browser/web_contents_permission_helper.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/browser/window_list.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/files/file_util.h"
#include "base/stl_util.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/string_util.h"
#include "chrome/browser/printing/printing_message_filter.h"
#include "chrome/browser/renderer_host/pepper/chrome_browser_pepper_host_factory.h"
#include "chrome/browser/renderer_host/pepper/widevine_cdm_message_filter.h"
#include "chrome/browser/speech/tts_message_filter.h"
#include "content/common/resource_request_body_impl.h"
#include "content/public/browser/browser_ppapi_host.h"