Make SkBlitter hierarchy explicit about what needs to be implemented.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2053823002

Review-Url: https://codereview.chromium.org/2053823002
diff --git a/tests/BlitMaskClip.cpp b/tests/BlitMaskClip.cpp
index 8ba2cea..21fe562 100644
--- a/tests/BlitMaskClip.cpp
+++ b/tests/BlitMaskClip.cpp
@@ -24,6 +24,10 @@
         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;