* 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
bool default_value) {
  bool bool_value = default_value;
  dict_.GetBoolean(name, &bool_value);
  return bool_value;
}
void WebContentsPreferences::Merge(const base::DictionaryValue& extend) {
  dict_.MergeDictionary(&extend);
}
content::WebContents* WebContentsPreferences::GetWebContentsFromProcessID(
    int process_id) {
  for (WebContentsPreferences* preferences : instances_) {
    content::WebContents* web_contents = preferences->web_contents_;
    if (web_contents->GetMainFrame()->GetProcess()->GetID() == process_id)
      return web_contents;
  }
  return nullptr;
}
WebContentsPreferences* WebContentsPreferences::From(
    content::WebContents* web_contents) {
  if (!web_contents)
    return nullptr;
  return FromWebContents(web_contents);
}
void WebContentsPreferences::AppendCommandLineSwitches(