The ornamental design for a chair, as shown and described.
FIG. 1 is a right front perspective view of a chair showing my new design. A left front perspective is omitted because it is a mirror image to the right front perspective view; and,
FIG. 2 is a right rear perspective view thereof. A left rear perspective is omitted because it is a mirror image to the left rear perspective view.
A bottom view of the claimed design is unornamented, and thus, has been omitted.