* 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
last_dict_ = std::move(*dict_.CreateDeepCopy());
}
void WebContentsPreferences::OverrideWebkitPrefs(
    content::WebPreferences* prefs) {
  bool b;
  if (dict_.GetBoolean("javascript", &b))
    prefs->javascript_enabled = b;
  if (dict_.GetBoolean("images", &b))
    prefs->images_enabled = b;
  if (dict_.GetBoolean("textAreasAreResizable", &b))
    prefs->text_areas_are_resizable = b;
  if (dict_.GetBoolean("webgl", &b)) {
    prefs->webgl1_enabled = b;
    prefs->webgl2_enabled = b;
  }
  if (dict_.GetBoolean("webSecurity", &b)) {
    prefs->web_security_enabled = b;
    prefs->allow_running_insecure_content = !b;
  }
  if (dict_.GetBoolean("allowRunningInsecureContent", &b))
    prefs->allow_running_insecure_content = b;
  const base::DictionaryValue* fonts = nullptr;
  if (dict_.GetDictionary("defaultFontFamily", &fonts)) {
    base::string16 font;
    if (fonts->GetString("standard", &font))
      prefs->standard_font_family_map[content::kCommonScript] = font;
    if (fonts->GetString("serif", &font))
      prefs->serif_font_family_map[content::kCommonScript] = font;
    if (fonts->GetString("sansSerif", &font))
      prefs->sans_serif_font_family_map[content::kCommonScript] = font;