factories should return baseclass, allowing the impl to specialize

bench/MorphologyBench.cpp

Index: bench/MorphologyBench.cpp
diff --git a/bench/MorphologyBench.cpp b/bench/MorphologyBench.cpp
index 51da197e152b779a3213def82f545692e6f1ada5..fd60ebed466b5f7bdb805d7784f196024bf5305b 100644
--- a/bench/MorphologyBench.cpp
+++ b/bench/MorphologyBench.cpp
@@ -63,7 +63,7 @@ protected:
             r.offset(fRadius, fRadius);
 
             if (fRadius > 0) {
-                SkMorphologyImageFilter* mf = nullptr;
+                SkImageFilter* mf = nullptr;
                 switch (fStyle) {
                 case kDilate_MT:
                     mf = SkDilateImageFilter::Create(SkScalarFloorToInt(fRadius),