CLAIM The ornamental design for a chair base, as shown and described.
FIG. 1 is a perspective view showing the top, opposite sides and front of a chair base which forms my new design, the broken lines of a chair and chair base parts being for illustrative purposes only and forming no part of my design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,