Get OffsetImageFilter really working with SkSpecialImages

include/effects/SkOffsetImageFilter.h

Index: include/effects/SkOffsetImageFilter.h
diff --git a/include/effects/SkOffsetImageFilter.h b/include/effects/SkOffsetImageFilter.h
index 04b896a956d36ad2130f8b684205c16c47c900a5..d10a53600176342e1103b067004982845eabe0d3 100644
--- a/include/effects/SkOffsetImageFilter.h
+++ b/include/effects/SkOffsetImageFilter.h
@@ -26,10 +26,15 @@ public:
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkOffsetImageFilter)
 
+    // TODO: remove this when all the image filters are converted
+    bool supportsUnifiedFiltering() const override { return true; }
+
 protected:
     void flatten(SkWriteBuffer&) const override;
     bool onFilterImageDeprecated(Proxy*, const SkBitmap& src, const Context&, SkBitmap* result,
                                  SkIPoint* loc) const override;
+    SkSpecialImage* onFilterImage(SkSpecialImage* src, const Context&,
+                                  SkIPoint* offset) const override;
     void onFilterNodeBounds(const SkIRect&, const SkMatrix&, SkIRect*, MapDirection) const override;
 
 private: