* 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
content::WebContents* web_contents,
    const mate::Dictionary& web_preferences)
    : web_contents_(web_contents) {
  v8::Isolate* isolate = web_preferences.isolate();
  mate::Dictionary copied(isolate, web_preferences.GetHandle()->Clone());
  
  copied.Delete("embedder");
  copied.Delete("isGuest");
  copied.Delete("session");
  mate::ConvertFromV8(isolate, copied.GetHandle(), &dict_);
  web_contents->SetUserData(UserDataKey(), base::WrapUnique(this));
  instances_.push_back(this);
  
  SetDefaultBoolIfUndefined("plugins", false);
  SetDefaultBoolIfUndefined(options::kExperimentalFeatures, false);
  SetDefaultBoolIfUndefined(options::kExperimentalCanvasFeatures, false);
  bool node = SetDefaultBoolIfUndefined(options::kNodeIntegration, true);
  SetDefaultBoolIfUndefined(options::kNodeIntegrationInWorker, false);
  SetDefaultBoolIfUndefined(options::kWebviewTag, node);