* 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
base::CommandLine* command_line) {
  bool b;
  
  if (dict_.GetBoolean("plugins", &b) && b)
    command_line->AppendSwitch(switches::kEnablePlugins);
  
  if (dict_.GetBoolean(options::kExperimentalFeatures, &b) && b)
    command_line->AppendSwitch(
        ::switches::kEnableExperimentalWebPlatformFeatures);
  if (dict_.GetBoolean(options::kExperimentalCanvasFeatures, &b) && b)
    command_line->AppendSwitch(::switches::kEnableExperimentalCanvasFeatures);
  
  bool node_integration = true;
  dict_.GetBoolean(options::kNodeIntegration, &node_integration);
  command_line->AppendSwitchASCII(switches::kNodeIntegration,
                                  node_integration ? "true" : "false");
  
  if (dict_.GetBoolean(options::kNodeIntegrationInWorker, &b) && b)
    command_line->AppendSwitch(switches::kNodeIntegrationInWorker);
  
  
  bool webview_tag = node_integration;
  dict_.GetBoolean(options::kWebviewTag, &webview_tag);
  command_line->AppendSwitchASCII(switches::kWebviewTag,
                                  webview_tag ? "true" : "false");