The ornamental design for a grass chuteblocker, as shown and described.
FIG. 1 is a front perspective view of a grass chute blocker showing my new design attached to the environmental broken line showing of a mower housing.
FIG. 2 is an enlarged perspective view thereof with the handle portion shown broken away for indeterminate length.
FIG. 3 is a back elavational view thereof.
FIG. 4 is a front elevational view thereof.
FIG. 5 is a side elevational view thereof.
FIG. 6 is an opposite side elevational view thereof.
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line figure in FIG. 1, depicting a section of the housing of a rotary mower, is an environmental element, and is not part of my new design.