Make SkBlitter hierarchy explicit about what needs to be implemented.

tests/BlitMaskClip.cpp

Index: tests/BlitMaskClip.cpp
diff --git a/tests/BlitMaskClip.cpp b/tests/BlitMaskClip.cpp
index 8ba2cea3e065316b114ad05f343bd2e9137a0a64..21fe56236850ba537be400965deea51e6f943467 100644
--- a/tests/BlitMaskClip.cpp
+++ b/tests/BlitMaskClip.cpp
@@ -24,6 +24,10 @@ public:
         REPORTER_ASSERT(fReporter, right > fBounds.fLeft && right <= fBounds.fRight);
     }
 
+    void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) override {
+        SkDEBUGFAIL("blitAntiH not implemented");
+    }
+
 private:
     SkIRect fBounds;
     skiatest::Reporter* fReporter;