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
auto getter = browser_context_->GetRequestContext();
  content::BrowserThread::PostTask(
      content::BrowserThread::IO, FROM_HERE,
      base::BindOnce(ClearJobFactoryInIO, base::RetainedRef(getter)));
  content::BrowserContext::GetDownloadManager(browser_context())->
      RemoveObserver(this);
  DestroyGlobalHandle(isolate(), cookies_);
  DestroyGlobalHandle(isolate(), web_request_);
  DestroyGlobalHandle(isolate(), protocol_);
  g_sessions.erase(weak_map_id());
}
void Session::OnDownloadCreated(content::DownloadManager* manager,
                                content::DownloadItem* item) {
  if (item->IsSavePackageDownload())
    return;
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto handle = DownloadItem::Create(isolate(), item);
  if (item->GetState() == content::DownloadItem::INTERRUPTED)
    handle->SetSavePath(item->GetTargetFilePath());
  bool prevent_default = Emit("will-download", handle, item->GetWebContents());
  if (prevent_default) {
    item->Cancel(true);
    item->Remove();
  }
}
void Session::ResolveProxy(const GURL& url, ResolveProxyCallback callback) {