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
content::BrowserContext::GetDownloadManager(browser_context())->
      RemoveObserver(this);
}
void Session::OnDownloadCreated(content::DownloadManager* manager,
                                content::DownloadItem* item) {
  if (item->IsSavePackageDownload())
    return;
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit(
      "will-download",
      DownloadItem::Create(isolate(), item),
      item->GetWebContents());
  if (prevent_default) {
    item->Cancel(true);
    item->Remove();
  }
}
void Session::ResolveProxy(const GURL& url, ResolveProxyCallback callback) {
  new ResolveProxyHelper(browser_context(), url, callback);
}
template<Session::CacheAction action>
void Session::DoCacheAction(const net::CompletionCallback& callback) {
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&DoCacheActionInIO,
                 make_scoped_refptr(browser_context_->GetRequestContext()),
                 action,
                 callback));
}
void Session::ClearStorageData(mate::Arguments* args) {