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,
    Session::CacheAction action,
    const net::CompletionCallback& callback) {
  auto request_context = context_getter->GetURLRequestContext();
  auto http_cache = request_context->http_transaction_factory()->GetCache();
  if (!http_cache)
    RunCallbackInUI<int>(callback, net::ERR_FAILED);
  
  using BackendPtr = disk_cache::Backend*;
  auto* backend_ptr = new BackendPtr(nullptr);
  net::CompletionCallback on_get_backend =
      base::Bind(&OnGetBackend, base::Owned(backend_ptr), action, callback);
  int rv = http_cache->GetBackend(backend_ptr, on_get_backend);
  if (rv != net::ERR_IO_PENDING)
    on_get_backend.Run(net::OK);
}
void SetProxyInIO(scoped_refptr<net::URLRequestContextGetter> getter,