The ornamental design for a boosterseat, as shown and described.
FIG. 1 is a front elevational view of a booster seat, showing my new
design, the broken lines are for environmental purposes only and form no
part of the claimed design;
FIG. 2 is a top plan view of the booster seat;
FIG. 3 is a side elevational view of the booster seat, the reverse side
being the same;
FIG. 4 is a bottom plan view of the booster seat; and,
FIG. 5 is an end elevational view of the booster seat, the reverse side
being the same.