Initial error handling code

include/effects/SkArithmeticMode.h

Index: include/effects/SkArithmeticMode.h
diff --git a/include/effects/SkArithmeticMode.h b/include/effects/SkArithmeticMode.h
index 9de332caef4016e414165549ae07b8eca8de6289..46b7121bb4c5863f0cf2d15a874b021f63aca96b 100644
--- a/include/effects/SkArithmeticMode.h
+++ b/include/effects/SkArithmeticMode.h
@@ -8,9 +8,12 @@
 #ifndef SkArithmeticMode_DEFINED
 #define SkArithmeticMode_DEFINED
 
-#include "SkXfermode.h"
+#include "SkFlattenable.h"
+#include "SkScalar.h"
 
-class SK_API SkArithmeticMode : public SkXfermode {
+class SkXfermode;
+
+class SK_API SkArithmeticMode {
 public:
     /**
      *  result = clamp[k1 * src * dst + k2 * src + k3 * dst + k4]
@@ -28,7 +31,7 @@ public:
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();
 
 private:
-    typedef SkXfermode INHERITED;
+    SkArithmeticMode(); // can't be instantiated
 };
 
 #endif