The ornamental design for a mask assembly, as shown and described.
FIG. 1 is a front perspective view of a mask assembly according to our new design;
FIG. 2 is a side view of the mask assembly thereof; and,
FIG. 3 is a front view of the mask assembly thereof.
This design is directed to a mask assembly. Where included, broken lines illustrate environmental structure of the mask assembly, which forms no part of the claimed design.