This change replaces all uses of `base::Singleton` across the codebase, in favor of `base::NoDestructor`. This is following the a direction taken upstream, in getting `base::Singleton` deleted. LSC doc: https://docs.google.com/document/d/1x1LqRQyfBOmpMkNQBYs7QBPSxLtuiImvmgcJYI_kaS4/edit?usp=sharing