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..c0039ec64a8b91be4b65dac3d2ea9372672e6eff 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -1438,6 +1438,10 @@ 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 {
@@ -1781,6 +1785,10 @@ bool SkXfermode::AsCoeff(const SkXfermode* xfer, Coeff* src, Coeff* dst) {
     return xfer->asCoeff(src, dst);
 }
 
+bool SkXfermode::IsValidCoeff(Coeff coeff) {
+    return coeff >= 0 && coeff < kCoeffCount;
+}
+
 bool SkXfermode::IsMode(const SkXfermode* xfer, Mode mode) {
     // if xfer==null then the mode is srcover
     Mode m = kSrcOver_Mode;
@@ -1790,6 +1798,10 @@ bool SkXfermode::IsMode(const SkXfermode* xfer, Mode mode) {
     return mode == m;
 }
 
+bool SkXfermode::IsValidMode(Mode mode) {
+    return (mode >= 0) && (mode <= SkXfermode::kLastMode);
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 //////////// 16bit xfermode procs