* 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
return true;
    }
  }
  
  auto src_url = current_instance->GetSiteURL();
  return
      !content::SiteInstance::IsSameWebSite(browser_context, src_url, url) &&
      
      
      
      content::SiteInstance::GetSiteForURL(browser_context, url) != src_url;
}
void AtomBrowserClient::AddProcessPreferences(
    int process_id, AtomBrowserClient::ProcessPreferences prefs) {
  base::AutoLock auto_lock(process_preferences_lock_);
  process_preferences_[process_id] = prefs;
}
void AtomBrowserClient::RemoveProcessPreferences(int process_id) {
  base::AutoLock auto_lock(process_preferences_lock_);
  process_preferences_.erase(process_id);
}
bool AtomBrowserClient::IsRendererSandboxed(int process_id) {
  base::AutoLock auto_lock(process_preferences_lock_);
  auto it = process_preferences_.find(process_id);
  return it != process_preferences_.end() && it->second.sandbox;
}
bool AtomBrowserClient::RendererUsesNativeWindowOpen(int process_id) {