The ornamental design for a bag for trolley, as shown and described.
FIG. 1 is an isometric view of a bag for a trolley according to an embodiment of the present invention, taken from the front and to the left of the bag; and,
FIG. 2 is an isometric view of the bag for a trolley shown in FIG. 1, taken from the rear and to the left of the bag. The portions of the bag not visible in FIGS. 1 and 2 form no part of the claim.