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* prefs = SessionPreferences::FromBrowserContext(browser_context());
  DCHECK(prefs);
  return prefs->preloads();
}
v8::Local<v8::Value> Session::Cookies(v8::Isolate* isolate) {
  if (cookies_.IsEmpty()) {
    auto handle = Cookies::Create(isolate, browser_context());
    cookies_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, cookies_);
}
v8::Local<v8::Value> Session::Protocol(v8::Isolate* isolate) {
  if (protocol_.IsEmpty()) {
    auto handle = atom::api::Protocol::Create(isolate, browser_context());
    protocol_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, protocol_);
}
v8::Local<v8::Value> Session::WebRequest(v8::Isolate* isolate) {
  if (web_request_.IsEmpty()) {
    auto handle = atom::api::WebRequest::Create(isolate, browser_context());
    web_request_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, web_request_);
}
mate::Handle<Session> Session::CreateFrom(