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 std::string& domains) {
  auto request_context = context_getter->GetURLRequestContext();
  auto auth_handler = request_context->http_auth_handler_factory();
  if (auth_handler) {
    auto auth_preferences = const_cast<net::HttpAuthPreferences*>(
        auth_handler->http_auth_preferences());
    if (auth_preferences)
      auth_preferences->set_server_whitelist(domains);
  }
}
void OnClearStorageDataDone(const base::Closure& callback) {