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
if (!callback.is_null())
    callback.Run();
}
void DownloadIdCallback(content::DownloadManager* download_manager,
                        const base::FilePath& path,
                        const std::vector<GURL>& url_chain,
                        const std::string& mime_type,
                        int64_t offset,
                        int64_t length,
                        const std::string& last_modified,
                        const std::string& etag,
                        const base::Time& start_time,
                        uint32_t id) {
  download_manager->CreateDownloadItem(
      base::GenerateGUID(), id, path, path, url_chain, GURL(), GURL(), GURL(),
      GURL(), mime_type, mime_type, start_time, base::Time(), etag,
      last_modified, offset, length, std::string(),
      content::DownloadItem::INTERRUPTED,
      content::DownloadDangerType::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS,
      content::DOWNLOAD_INTERRUPT_REASON_NETWORK_TIMEOUT, false,
      base::Time(), false,
      std::vector<content::DownloadItem::ReceivedSlice>());
}
void SetDevToolsNetworkEmulationClientIdInIO(