This is an umbrella bug to track the progress of the migration.
Bind(), BindOnce() and BindRepeating() create Callback, OnceCallback, and RepeatingCallback respectively.
Bind() and Callback are currently used. BindOnce() and OnceCallback are next-gen. BindRepeating() and RepeatingCallback are aliases to Bind() and Callback.
See https://chromium.googlesource.com/chromium/src/+/master/docs/callback.md for details.
OnceCallback is move-only, and can be called at most once (clobbered on the first call).
As it has clear lifetime and better move-only type handling, I'm proposing to migrate most of Bind/Callback to BindOnce/OnceCallback.
We already have BindOnce/OnceCallback implementation, PostTask impls can take OnceCallback, Mojo code generator has use_once_callback flag to switch the interface, and some clang-based refactoring tools are in //tools/clang. So, I consider it's ready to start the mass migration.