Make SkBlitter hierarchy explicit about what needs to be implemented.

src/core/SkRegion_path.cpp

Index: src/core/SkRegion_path.cpp
diff --git a/src/core/SkRegion_path.cpp b/src/core/SkRegion_path.cpp
index 9289641c69d5eab494c37d78f5ba2726e13c1114..47df8f6d71fbf4c49a396f60a030d2126edc8a09 100644
--- a/src/core/SkRegion_path.cpp
+++ b/src/core/SkRegion_path.cpp
@@ -45,6 +45,9 @@ public:
     void    copyToRgn(SkRegion::RunType runs[]) const;
 
     void blitH(int x, int y, int width) override;
+    void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) override {
+        SkDEBUGFAIL("blitAntiH not implemented");
+    }
 
 #ifdef SK_DEBUG
     void dump() const {