* 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 "content/public/browser/client_certificate_delegate.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/resource_dispatcher_host.h"
#include "content/public/browser/site_instance.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/content_switches.h"
#include "content/public/common/url_constants.h"
#include "content/public/common/web_preferences.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "ppapi/host/ppapi_host.h"
#include "ui/base/l10n/l10n_util.h"
#include "v8/include/v8.h"
namespace atom {
namespace {
bool g_suppress_renderer_process_restart = false;
std::string g_custom_service_worker_schemes = "";
void Noop(scoped_refptr<content::SiteInstance>) {
}
}  
void AtomBrowserClient::SuppressRendererProcessRestartForOnce() {
  g_suppress_renderer_process_restart = true;
}
void AtomBrowserClient::SetCustomServiceWorkerSchemes(
    const std::vector<std::string>& schemes) {
  g_custom_service_worker_schemes = base::JoinString(schemes, ",");
}
AtomBrowserClient::AtomBrowserClient() : delegate_(nullptr) {
}
AtomBrowserClient::~AtomBrowserClient() {