The ornamental design for a washtub for a wheelbarrow, as shown and described.
FIG. 1 is a top perspective view of a wash tub for a wheelbarrow;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a front elevation thereof;
FIG. 5 is a left side elevation thereof;
FIG. 6 is a right side elevation thereof;
FIG. 7 is a rear elevation thereof; and,
FIG. 8 is a bottom plan view thereof.
The steady broken lines in the drawings are for the purpose of illustrating portions of the wash tub for a wheelbarrow that form no part of the claimed design. The dot-dash broken lines in the drawings are for the purpose of illustrating the bounds of the claimed design.