The ornamental design for chair, as shown and described.
FIG. 1 is a left side elevation view of the chair showing my new design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line showing of portions of the chair is included for the purpose of illustrating environmental structure and forms no part of the claimed design.