Make SkBlitter hierarchy explicit about what needs to be implemented.

tests/BlitMaskClip.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlitter.h"
 #include "SkMask.h"
 #include "Test.h"
 #include <string.h>
 
 class TestBlitter : public SkBlitter {
 public:
     TestBlitter(SkIRect bounds, skiatest::Reporter* reporter)
         : fBounds(bounds)
         , fReporter(reporter) { }
 
     void blitH(int x, int y, int width) override {
 
         REPORTER_ASSERT(fReporter, x >= fBounds.fLeft && x < fBounds.fRight);
         REPORTER_ASSERT(fReporter, y >= fBounds.fTop && y < fBounds.fBottom);
         int right = x + width;
         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;
 };
 
 // Exercise all clips compared with different widths of bitMask. Make sure that no buffer
 // overruns happen.
 DEF_TEST(BlitAndClip, reporter) {
     const int originX = 100;
     const int originY = 100;
@@ skipping 20 matching lines @@
                         SkIRect clipRect = {left, top, right, bottom};
                         tb.blitMask(mask, clipRect);
                     }
                 }
             }
         }
 
         delete [] bits;
     }
 }