DataGrid.load_extra_state() allows a subclass to load state from a

profile into the grid, or to store data into the profile. If the

subclass makes use of profile objects, and has modifications to make, it

has historically returned a boolean value, which indicates whether the

object should be saved.
This save operation runs the risk of overriding fields set elsewhere,

since it's an all-or-nothing. That can have many side-effects.
To avoid this, load_extra_state() is now expected to return a list of

fields that were modified on the provided profile. This can be an empty

list (equivalent of False before). DataGrid will take the list and

pass it to save(update_fields=...), minimizing the changes written to

the database.
The old boolean support still exists for now, but will be removed in

Djblets 4.0.

Unit tests pass.
Made use of this in the equivalent change for Review Board.