The ornamental design for a handheldinfantmassager, as shown and described.
FIG. 1 is a front elevational view of a handheld infant massager showing my new design;
FIG. 2 is a top front perspective view thereof;
FIG. 3 is a bottom rear perspective view thereof; and,
FIG. 4 is a bottom front perspective view thereof.
The broken lines shown in the drawings are for the purpose of illustrating portions of the handheld infant massager, which form no part of the claimed design.