The ornamental design for a portablewashingbasin, as shown and described.
FIG. 1 is a top front right side perspective of a portable washing basin
showing my new design, the left side being a mirror thereof; and,
FIG. 2 is a bottom rear left side perspective thereof, the right side being
a mirror thereof. The broken line showing is for illustrative purposes
only and forms no part of the claimed design.