>I think Chromium base classes contain something to create singleton (base::Singleton class as far as I remember)

@pozdnyakov Thank you for your suggestion. I will check how to use base::Singleton.
>Optional: I wonder why we need a singleton in the first place. Hanging the needed object in the XWalKExtension subclass doesn't suffice?

@cmarcelo Due to we only need one set of CPU, Display, Memory, Storage to query the info what we need. Here, I use singleton to limit these classes. It's also fine if we don't use it. If you think there is any disadvantage, we can change it in future.