The ornamental design for a chair accessory, as shown and described.
FIG. 1 is a first perspective view of a chair accessory showing our new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a side view thereof, the opposite side being a mirror image;
FIG. 6 is a section view thereof taken along the section line indicated in FIG. 4; and,
FIG. 7 is an enlarged view of the top of the section view of FIG. 6.
The uniform broken lines shown in the drawings are included for purposes of illustrating portions of the chair accessory and form no part of the claimed design; whereas the non-uniform broken lines shown in the drawings are included for purposes of illustrating environment and form no part of the claimed design.