Initial error handling code

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 1efd9efc4b2eae7a7ba7020e99a88d28e9e963f2..a0d960d3a001d39394c5af27448259234f8dcff2 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -12,6 +12,7 @@
 #include "SkFlattenableBuffers.h"
 #include "SkMathPriv.h"
 #include "SkString.h"
+#include "SkValidation.h"
 
 SK_DEFINE_INST_COUNT(SkXfermode)
 
@@ -1438,6 +1439,10 @@ protected:
         fDstCoeff = rec.fDC;
         // now update our function-ptr in the super class
         this->INHERITED::setProc(rec.fProc);
+
+        buffer.validateData(IsValidMode(fMode) &&
+                            IsValidCoeff(fSrcCoeff) &&
+                            IsValidCoeff(fDstCoeff));
     }
 
     virtual void flatten(SkFlattenableWriteBuffer& buffer) const SK_OVERRIDE {