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
public:
  ResolveProxyHelper(AtomBrowserContext* browser_context,
                     const GURL& url,
                     const Session::ResolveProxyCallback& callback)
      : callback_(callback),
        original_thread_(base::ThreadTaskRunnerHandle::Get()) {
    scoped_refptr<net::URLRequestContextGetter> context_getter =
        browser_context->url_request_context_getter();
    context_getter->GetNetworkTaskRunner()->PostTask(
        FROM_HERE,
        base::Bind(&ResolveProxyHelper::ResolveProxy,
                   base::Unretained(this), context_getter, url));
  }
  void OnResolveProxyCompleted(int result) {
    std::string proxy;
    if (result == net::OK)
      proxy = proxy_info_.ToPacString();
    original_thread_->PostTask(FROM_HERE,
                               base::Bind(callback_, proxy));
    delete this;
  }
 private:
  void ResolveProxy(scoped_refptr<net::URLRequestContextGetter> context_getter,