Switch SkDownSampleImageFilter over to new onFilterImage interface

include/effects/SkTestImageFilters.h

Index: include/effects/SkTestImageFilters.h
diff --git a/include/effects/SkTestImageFilters.h b/include/effects/SkTestImageFilters.h
index 2b3ca84da8a4e12d5c34156846c5323bd5bdba53..943bb533562d03cbbfb2cee9151e69ef1a73985a 100644
--- a/include/effects/SkTestImageFilters.h
+++ b/include/effects/SkTestImageFilters.h
@@ -30,8 +30,9 @@ public:
 
 protected:
     void flatten(SkWriteBuffer&) const override;
-    bool onFilterImageDeprecated(Proxy*, const SkBitmap& src, const Context&, SkBitmap* result,
-                                 SkIPoint* loc) const override;
+
+    sk_sp<SkSpecialImage> onFilterImage(SkSpecialImage* source, const Context&,
+                                        SkIPoint* offset) const override;
 
 private:
     SkDownSampleImageFilter(SkScalar scale, SkImageFilter* input)