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
const v8::Global<v8::Value>& global_handle) {
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  if (!global_handle.IsEmpty()) {
    v8::Local<v8::Value> local_handle = global_handle.Get(isolate);
    if (local_handle->IsObject()) {
      v8::Local<v8::Object> object = local_handle->ToObject();
      void* ptr = object->GetAlignedPointerFromInternalField(0);
      if (!ptr)
        return;
      delete static_cast<mate::WrappableBase*>(ptr);
      object->SetAlignedPointerInInternalField(0, nullptr);
    }
  }
}
}  
Session::Session(v8::Isolate* isolate, AtomBrowserContext* browser_context)
    : devtools_network_emulation_client_id_(base::GenerateGUID()),
      browser_context_(browser_context) {
  
  content::BrowserContext::GetDownloadManager(browser_context)->
      AddObserver(this);
  new SessionPreferences(browser_context);
  Init(isolate);
  AttachAsUserData(browser_context);
}
Session::~Session() {