I claim the ornamental design for a lounge chair, as shown and described.
FIG. 1 is a front right perspective view of a lounge chair of Applicant's design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a first side elevational view thereof, the second side elevational view opposite the first side elevational view and being a mirror image thereof;