* 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::BrowserPpapiHost* host) {
  host->GetPpapiHost()->AddHostFactoryFilter(
      base::WrapUnique(new chrome::ChromeBrowserPepperHostFactory(host)));
}
content::QuotaPermissionContext*
    AtomBrowserClient::CreateQuotaPermissionContext() {
  return new AtomQuotaPermissionContext;
}
void AtomBrowserClient::AllowCertificateError(
    content::WebContents* web_contents,
    int cert_error,
    const net::SSLInfo& ssl_info,
    const GURL& request_url,
    content::ResourceType resource_type,
    bool overridable,
    bool strict_enforcement,
    bool expired_previous_decision,
    const base::Callback<void(content::CertificateRequestResultType)>&
        callback) {
  if (delegate_) {
    delegate_->AllowCertificateError(
        web_contents, cert_error, ssl_info, request_url,
        resource_type, overridable, strict_enforcement,
        expired_previous_decision, callback);
  }
}
void AtomBrowserClient::SelectClientCertificate(