Make SkBlitter hierarchy explicit about what needs to be implemented.

src/core/SkSpriteBlitter.h

Index: src/core/SkSpriteBlitter.h
diff --git a/src/core/SkSpriteBlitter.h b/src/core/SkSpriteBlitter.h
index 62c50c8b35c267f3e1461ff2468e77dd9d48bcf6..3a62860867901c4075a47e3e2d37104c254ae0da 100644
--- a/src/core/SkSpriteBlitter.h
+++ b/src/core/SkSpriteBlitter.h
@@ -15,18 +15,23 @@
 
 class SkPaint;
 
+// SkSpriteBlitter specializes SkBlitter in a way to move large rectangles of pixels around.
+// Because of this use, the main primitive shifts from blitH style things to the more efficient
+// blitRect.
 class SkSpriteBlitter : public SkBlitter {
 public:
     SkSpriteBlitter(const SkPixmap& source);
 
     virtual void setup(const SkPixmap& dst, int left, int top, const SkPaint&);
 
-#ifdef SK_DEBUG
+    // blitH, blitAntiH, blitV and blitMask should not be called on an SkSpriteBlitter.
     void blitH(int x, int y, int width) override;
     void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) override;
     void blitV(int x, int y, int height, SkAlpha alpha) override;
     void blitMask(const SkMask&, const SkIRect& clip) override;
-#endif
+
+    // A SkSpriteBlitter must implement blitRect.
+    void blitRect(int x, int y, int width, int height) override = 0;
 
     static SkSpriteBlitter* ChooseD16(const SkPixmap& source, const SkPaint&, SkTBlitterAllocator*);
     static SkSpriteBlitter* ChooseL32(const SkPixmap& source, const SkPaint&, SkTBlitterAllocator*);
@@ -38,6 +43,9 @@ protected:
     const SkPixmap  fSource;
     int             fLeft, fTop;
     const SkPaint*  fPaint;
+
+private:
+    typedef SkBlitter INHERITED;
 };
 
 #endif