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
browser_context_->prefs()->SetFilePath(
      prefs::kDownloadDefaultDirectory, path);
}
void Session::EnableNetworkEmulation(const mate::Dictionary& options) {
  std::unique_ptr<content::DevToolsNetworkConditions> conditions;
  bool offline = false;
  double latency = 0.0, download_throughput = 0.0, upload_throughput = 0.0;
  if (options.Get("offline", &offline) && offline) {
    conditions.reset(new content::DevToolsNetworkConditions(offline));
  } else {
    options.Get("latency", &latency);
    options.Get("downloadThroughput", &download_throughput);
    options.Get("uploadThroughput", &upload_throughput);
    conditions.reset(new content::DevToolsNetworkConditions(
        false, latency, download_throughput, upload_throughput));
  }
  content::DevToolsNetworkController::SetNetworkState(
      devtools_network_emulation_client_id_, std::move(conditions));
  BrowserThread::PostTask(
      BrowserThread::IO, FROM_HERE,
      base::Bind(
          &SetDevToolsNetworkEmulationClientIdInIO,
          base::RetainedRef(browser_context_->url_request_context_getter()),
          devtools_network_emulation_client_id_));
}
void Session::DisableNetworkEmulation() {