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
brightray::URLRequestContextGetter* url_request_context_getter,
    const std::string& client_id) {
  if (!url_request_context_getter)
    return;
  net::URLRequestContext* context =
      url_request_context_getter->GetURLRequestContext();
  AtomNetworkDelegate* network_delegate =
      static_cast<AtomNetworkDelegate*>(context->network_delegate());
  network_delegate->SetDevToolsNetworkEmulationClientId(client_id);
}
void ClearJobFactoryInIO(
    scoped_refptr<brightray::URLRequestContextGetter> request_context_getter) {
  auto job_factory = static_cast<AtomURLRequestJobFactory*>(
      request_context_getter->job_factory());
  if (job_factory)
    job_factory->Clear();
}
void DestroyGlobalHandle(v8::Isolate* isolate,