* 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
content::WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    std::unique_ptr<content::ClientCertificateDelegate> delegate) {
  if (!cert_request_info->client_certs.empty() && delegate_) {
    delegate_->SelectClientCertificate(
        web_contents, cert_request_info, std::move(delegate));
  }
}
void AtomBrowserClient::ResourceDispatcherHostCreated() {
  resource_dispatcher_host_delegate_.reset(
      new AtomResourceDispatcherHostDelegate);
  content::ResourceDispatcherHost::Get()->SetDelegate(
      resource_dispatcher_host_delegate_.get());
}
bool AtomBrowserClient::CanCreateWindow(