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
BrowserThread::PostTask(
      BrowserThread::UI, FROM_HERE, base::Bind(callback, result...));
}
void OnGetBackend(disk_cache::Backend** backend_ptr,
                  Session::CacheAction action,
                  const net::CompletionCallback& callback,
                  int result) {
  if (result != net::OK) {
    RunCallbackInUI(callback, result);
  } else if (backend_ptr && *backend_ptr) {
    if (action == Session::CacheAction::CLEAR) {
      (*backend_ptr)->DoomAllEntries(base::Bind(&RunCallbackInUI<int>,
                                                callback));
    } else if (action == Session::CacheAction::STATS) {
      base::StringPairs stats;
      (*backend_ptr)->GetStats(&stats);
      for (const auto& stat : stats) {
        if (stat.first == "Current size") {
          int current_size;
          base::StringToInt(stat.second, &current_size);
          RunCallbackInUI(callback, current_size);
          break;
        }
      }
    }
  } else {
    RunCallbackInUI<int>(callback, net::ERR_FAILED);
  }
}
void DoCacheActionInIO(