We claim the ornamental design for a chair base, as shown and described.
FIG. 1 is a perspective view of the chair base;
FIG. 2 is a front side elevational view thereof along a leg of the base, the side elevational views along the four remaining legs being a copy of this view;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a side elevational view thereof facing an opening between two legs of the base, the side elevational views facing the remaining four openings being a copy of this view.
The shading illustrated in the figures is provided merely to represent the contour of the design and is not intended to be illustrative of texture, gloss, or surface decoration.