Steps:
1. Put some things in cache storage
2. Call `session.clearStorageData`

Expected:
Some way of clearing cache storage data

Actual:
No way of clearing cache storage data

This seems like it might just be missing an option, in `atom/browser/api/atom_api_session.cc#GetStorageMask` there are only options for some storages. What's missing is a way of setting any of these:
```
REMOVE_DATA_MASK_CACHE_STORAGE
REMOVE_DATA_MASK_PLUGIN_PRIVATE_DATA
```

* Electron version: 1.8.2
* Operating system: macos 10.13.3

Note: I didn't make a test app for this because it seems like the issue is obvious, let me know if you'd line one though.
atom/browser/api/atom_api_session.cc
const GURL& url) {
    DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
    net::ProxyService* proxy_service =
        context_getter->GetURLRequestContext()->proxy_service();
    net::CompletionCallback completion_callback =
        base::Bind(&ResolveProxyHelper::OnResolveProxyCompleted,
                   base::Unretained(this));
    
    int result = proxy_service->ResolveProxy(
        url, "GET", &proxy_info_, completion_callback, &pac_req_, nullptr,
        net::NetLogWithSource());
    
    if (result != net::ERR_IO_PENDING)
      completion_callback.Run(result);
  }
  Session::ResolveProxyCallback callback_;
  net::ProxyInfo proxy_info_;
  net::ProxyService::PacRequest* pac_req_;
  scoped_refptr<base::SingleThreadTaskRunner> original_thread_;
  DISALLOW_COPY_AND_ASSIGN(ResolveProxyHelper);
};
void RunCallbackInUI(const base::Callback<void()>& callback) {
  BrowserThread::PostTask(BrowserThread::UI, FROM_HERE, callback);
}
template<typename ...T>
void RunCallbackInUI(const base::Callback<void(T...)>& callback, T... result) {