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
const scoped_refptr<net::URLRequestContextGetter>& context_getter,
    Session::CacheAction action,
    const net::CompletionCallback& callback) {
  auto request_context = context_getter->GetURLRequestContext();
  auto http_cache = request_context->http_transaction_factory()->GetCache();
  if (!http_cache)
    RunCallbackInUI<int>(callback, net::ERR_FAILED);
  
  using BackendPtr = disk_cache::Backend*;
  auto* backend_ptr = new BackendPtr(nullptr);
  net::CompletionCallback on_get_backend =
      base::Bind(&OnGetBackend, base::Owned(backend_ptr), action, callback);
  int rv = http_cache->GetBackend(backend_ptr, on_get_backend);
  if (rv != net::ERR_IO_PENDING)
    on_get_backend.Run(net::OK);
}
void SetProxyInIO(net::URLRequestContextGetter* getter,