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
.SetMethod("getPreloads", &Session::GetPreloads)
      .SetProperty("cookies", &Session::Cookies)
      .SetProperty("protocol", &Session::Protocol)
      .SetProperty("webRequest", &Session::WebRequest);
}
}  
}  
namespace {
using atom::api::Session;
v8::Local<v8::Value> FromPartition(
    const std::string& partition, mate::Arguments* args) {
  if (!atom::Browser::Get()->is_ready()) {
    args->ThrowError("Session can only be received when app is ready");
    return v8::Null(args->isolate());
  }
  base::DictionaryValue options;
  args->GetNext(&options);
  return Session::FromPartition(args->isolate(), partition, options).ToV8();
}
void Initialize(v8::Local<v8::Object> exports, v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context, void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  mate::Dictionary dict(isolate, exports);
  dict.Set("Session", Session::GetConstructor(isolate)->GetFunction());
  dict.Set("Cookies", Cookies::GetConstructor(isolate)->GetFunction());
  dict.SetMethod("fromPartition", &FromPartition);
}
}  
NODE_BUILTIN_MODULE_CONTEXT_AWARE(atom_browser_session, Initialize)