* 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("sandbox", &b) && b)
    command_line->AppendSwitch(switches::kEnableSandbox);
  else if (!command_line->HasSwitch(switches::kEnableSandbox))
    command_line->AppendSwitch(::switches::kNoSandbox);
  if (dict_.GetBoolean("nativeWindowOpen", &b) && b)
    command_line->AppendSwitch(switches::kNativeWindowOpen);
  
  base::FilePath::StringType preload;
  if (dict_.GetString(options::kPreloadScript, &preload)) {
    if (base::FilePath(preload).IsAbsolute())
      command_line->AppendSwitchNative(switches::kPreloadScript, preload);
    else
      LOG(ERROR) << "preload script must have absolute path.";
  } else if (dict_.GetString(options::kPreloadURL, &preload)) {
    
    base::FilePath preload_path;
    if (net::FileURLToFilePath(GURL(preload), &preload_path))
      command_line->AppendSwitchPath(switches::kPreloadScript, preload_path);
    else
      LOG(ERROR) << "preload url must be file:// protocol.";
  }
  
  base::Value* customArgs;
  if (dict_.Get(options::kCustomArgs, &customArgs) &&
      customArgs->is_list()) {
    for (const base::Value& customArg : customArgs->GetList()) {
      if (customArg.is_string())
        command_line->AppendArg(customArg.GetString());
    }
  }