FIG. 1 is a perspective view of the chair back cushion showing our new design, the broken line showing of the chair is for illustrative purposes only and forms no part of the claimed design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a left side view thereof with the right side being a mirror image of the left side;

FIG. 4 is a top plan view thereof;

FIG. 5 is a bottom plan view thereof; and;

FIG. 6 is a rear view thereof.