Docs:
https://github.com/electron/electron/blob/master/docs/api/session.md#sesclearstoragedataoptions-callback

I was running into a strange error when calling the `clearStorageData` method. It was saying it was unable to process the argument at index 1, which I was not passing. After some research I found that the callback is currently required, but in my case I did not care to provide one. To fix it, I supplied an empty function. Here is my code:

```
Electron.remote.getCurrentWebContents().session.clearStorageData(
    {
        storages: [
            'appcache',
            'cookies',
            'filesystem',
            'indexdb',
            'localstorage',
            'shadercache',
            'websql',
            'serviceworkers',
        ],
        quotas: [
            'temporary',
            'persistent',
            'syncable',
        ],
    },
    function() {} // <--- I shouldn't have to do this
);
```
- Electron version: 1.2.7
- Operating system: macOS

atom/browser/api/atom_api_session.cc
std::string proxy_rules;
    GURL pac_url;
    mate::Dictionary options;
    
    
    if (ConvertFromV8(isolate, val, &proxy_rules)) {
      pac_url = GURL(proxy_rules);  
    } else if (ConvertFromV8(isolate, val, &options)) {
      options.Get("pacScript", &pac_url);
      options.Get("proxyRules", &proxy_rules);
    } else {
      return false;
    }
    
    if (!pac_url.is_empty() && pac_url.is_valid()) {
      out->set_pac_url(pac_url);
    } else {
      out->proxy_rules().ParseFromString(proxy_rules);
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
const char kPersistPrefix[] = "persist:";
using WrapSessionCallback = base::Callback<void(v8::Local<v8::Value>)>;
WrapSessionCallback g_wrap_session;
class ResolveProxyHelper {