The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof, a left side elevational view not being shown as it is a mirror image of the right side elevational view; and,