* 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
if (dict_.GetBoolean(options::kContextIsolation, &b) && b)
    command_line->AppendSwitch(switches::kContextIsolation);
  
  std::string s;
  if (dict_.GetString(options::kBackgroundColor, &s))
    command_line->AppendSwitchASCII(switches::kBackgroundColor, s);
  
  int guest_instance_id = 0;
  if (dict_.GetInteger(options::kGuestInstanceID, &guest_instance_id))
    command_line->AppendSwitchASCII(switches::kGuestInstanceID,
                                    base::IntToString(guest_instance_id));
  
  int opener_id;
  if (dict_.GetInteger(options::kOpenerID, &opener_id))
    command_line->AppendSwitchASCII(switches::kOpenerID,
                                    base::IntToString(opener_id));
#if defined(OS_MACOSX)
  
  if (dict_.GetBoolean(options::kScrollBounce, &b) && b)
    command_line->AppendSwitch(switches::kScrollBounce);
#endif
  
  const base::ListValue* args;
  if (dict_.GetList("commandLineSwitches", &args)) {
    for (size_t i = 0; i < args->GetSize(); ++i) {
      std::string arg;
      if (args->GetString(i, &arg) && !arg.empty())
        command_line->AppendSwitch(arg);
    }