The ornamental design for an extendablecomfortheadrest, as shown and described.
FIG. 1 is a front elevational view of an extendable comfort headrest in a first position;
FIG. 2 is a rear elevational view thereof in the first position;
FIG. 3 is a right side elevational view thereof in the first position, the left side elevational view thereof in the first position being a mirror image of the right side elevational view in the first position;
FIG. 4 is a top plan view thereof in the first position;
FIG. 5 is a bottom plan view thereof in the first position;
FIG. 6 is a front perspective view thereof in the first position;
FIG. 7 is a front elevational view thereof in a second position; and,
FIG. 8 is a top plan view thereof in the second position.
The broken lines in the drawings depict environmental subject matter only and form no part of the claimed design.