We claim the ornamental design for a handheld electronic device, as shown and described.
FIG. 1 is a front perspective view of a handheld electronic device; and,
FIG. 2 is a front elevational view of the handheld electronic device.
The broken lines immediately surrounding the keys of the electronic device are understood to represent boundary lines, and all other broken lines are shown for the purpose of illustrating portions of the device and form no part of the claimed design.