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 scoped_refptr<net::URLRequestContextGetter>& context_getter,
    const ClearAuthCacheOptions& options,
    const base::Closure& callback) {
  auto request_context = context_getter->GetURLRequestContext();
  auto network_session =
      request_context->http_transaction_factory()->GetSession();
  if (network_session) {
    if (options.type == "password") {
      auto auth_cache = network_session->http_auth_cache();
      if (!options.origin.is_empty()) {
        auth_cache->Remove(
            options.origin, options.realm, options.auth_scheme,
            net::AuthCredentials(options.username, options.password));
      } else {
        auth_cache->ClearEntriesAddedWithin(base::TimeDelta::Max());
      }
    } else if (options.type == "clientCertificate") {
      auto client_auth_cache = network_session->ssl_client_auth_cache();
      client_auth_cache->Remove(net::HostPortPair::FromURL(options.origin));
    }
    network_session->CloseAllConnections();
  }
  if (!callback.is_null())
    RunCallbackInUI(callback);
}
void AllowNTLMCredentialsForDomainsInIO(