* 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
base::AutoLock auto_lock(process_preferences_lock_);
  auto it = process_preferences_.find(process_id);
  return it != process_preferences_.end() && it->second.native_window_open;
}
bool AtomBrowserClient::RendererDisablesPopups(int process_id) {
  base::AutoLock auto_lock(process_preferences_lock_);
  auto it = process_preferences_.find(process_id);
  return it != process_preferences_.end() && it->second.disable_popups;
}
void AtomBrowserClient::RenderProcessWillLaunch(
    content::RenderProcessHost* host) {
  int process_id = host->GetID();
  host->AddFilter(new printing::PrintingMessageFilter(process_id));
  host->AddFilter(new TtsMessageFilter(process_id, host->GetBrowserContext()));
  host->AddFilter(
      new WidevineCdmMessageFilter(process_id, host->GetBrowserContext()));
  content::WebContents* web_contents = GetWebContentsFromProcessID(process_id);
  ProcessPreferences process_prefs;
  process_prefs.sandbox = WebContentsPreferences::IsSandboxed(web_contents);
  process_prefs.native_window_open
      = WebContentsPreferences::UsesNativeWindowOpen(web_contents);
  process_prefs.disable_popups
      = WebContentsPreferences::DisablePopups(web_contents);
  AddProcessPreferences(host->GetID(), process_prefs);
  
  host->AddObserver(this);
}
content::SpeechRecognitionManagerDelegate*