* 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
SetDefaultBoolIfUndefined("sandbox", false);
  SetDefaultBoolIfUndefined("nativeWindowOpen", false);
  SetDefaultBoolIfUndefined(options::kContextIsolation, false);
  SetDefaultBoolIfUndefined("javascript", true);
  SetDefaultBoolIfUndefined("images", true);
  SetDefaultBoolIfUndefined("textAreasAreResizable", true);
  SetDefaultBoolIfUndefined("webgl", true);
  SetDefaultBoolIfUndefined("webSecurity", true);
  SetDefaultBoolIfUndefined("allowRunningInsecureContent", false);
  #if defined(OS_MACOSX)
  SetDefaultBoolIfUndefined(options::kScrollBounce, false);
  #endif
  SetDefaultBoolIfUndefined("offscreen", false);
  last_dict_ = std::move(*dict_.CreateDeepCopy());
}
WebContentsPreferences::~WebContentsPreferences() {
  instances_.erase(
      std::remove(instances_.begin(), instances_.end(), this),
      instances_.end());
}
bool WebContentsPreferences::SetDefaultBoolIfUndefined(
    const base::StringPiece& key, bool val) {
  bool existing;
  if (!dict_.GetBoolean(key, &existing)) {
    dict_.SetBoolean(key, val);
    return val;
  }
  return existing;
}
bool WebContentsPreferences::IsEnabled(const base::StringPiece& name,