* Electron version: 2.0.0-beta.7
* Operating system: Windows 7

### Expected behavior

`new BrowserWindow({webPreferences:{webSecurity:false}})` should not show mixed-content errors. It should show warnings and continue to load the content.

### Actual behavior

mixed-content errors are shown unless *allowRunningInsecureContent* is specified which is contrary to the documentation:

> Disabling webSecurity will disable the same-origin policy and set allowRunningInsecureContent property to true. 

This was working as expected in 1.8.2

### How to reproduce

Open a new BrowserWindow as above and load a page via `https` that contains elements using `http`.
atom/browser/web_contents_preferences.cc
}
  
  if (dict_.GetString(options::kBlinkFeatures, &s))
    command_line->AppendSwitchASCII(::switches::kEnableBlinkFeatures, s);
  
  if (dict_.GetString(options::kDisableBlinkFeatures, &s))
    command_line->AppendSwitchASCII(::switches::kDisableBlinkFeatures, s);
  if (guest_instance_id) {
    
    
    auto* manager = WebViewManager::GetWebViewManager(web_contents_);
    if (manager) {
      auto* embedder = manager->GetEmbedder(guest_instance_id);
      if (embedder) {
        auto* relay = NativeWindowRelay::FromWebContents(embedder);
        if (relay) {
          auto* window = relay->window.get();
          if (window) {
            const bool visible = window->IsVisible() && !window->IsMinimized();
            if (!visible) {
              command_line->AppendSwitch(switches::kHiddenPage);
            }
          }
        }
      }
    }
  }