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
mate::Arguments* args) {
  browser_context_->SetUserAgent(user_agent);
  std::string accept_lang = g_browser_process->GetApplicationLocale();
  args->GetNext(&accept_lang);
  scoped_refptr<brightray::URLRequestContextGetter> getter(
      browser_context_->GetRequestContext());
  getter->GetNetworkTaskRunner()->PostTask(
      FROM_HERE,
      base::Bind(&SetUserAgentInIO, getter, accept_lang, user_agent));
}
std::string Session::GetUserAgent() {
  return browser_context_->GetUserAgent();
}
void Session::GetBlobData(
    const std::string& uuid,
    const AtomBlobReader::CompletionCallback& callback) {
  if (callback.is_null())
    return;
  AtomBlobReader* blob_reader =
      browser_context()->GetBlobReader();
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&AtomBlobReader::StartReading,
                 base::Unretained(blob_reader),
                 uuid,
                 callback));
}
void Session::CreateInterruptedDownload(const mate::Dictionary& options) {