* 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
int opener_render_process_id,
    int opener_render_frame_id,
    const GURL& opener_url,
    const GURL& opener_top_level_frame_url,
    const GURL& source_origin,
    content::mojom::WindowContainerType container_type,
    const GURL& target_url,
    const content::Referrer& referrer,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const blink::mojom::WindowFeatures& features,
    const std::vector<std::string>& additional_features,
    const scoped_refptr<content::ResourceRequestBodyImpl>& body,
    bool user_gesture,
    bool opener_suppressed,
    content::ResourceContext* context,
    bool* no_javascript_access) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
  if (IsRendererSandboxed(opener_render_process_id)) {
    *no_javascript_access = false;
    return true;
  }
  if (RendererUsesNativeWindowOpen(opener_render_process_id)) {
    if (RendererDisablesPopups(opener_render_process_id)) {