Add mixed samples support to XPs

src/effects/SkArithmeticMode_gpu.h

Index: src/effects/SkArithmeticMode_gpu.h
diff --git a/src/effects/SkArithmeticMode_gpu.h b/src/effects/SkArithmeticMode_gpu.h
index 3334e2dce061cc023807b4562710bb6097eaa985..b9235b3813933faf52b7b9f9c2f9368b2a549139 100644
--- a/src/effects/SkArithmeticMode_gpu.h
+++ b/src/effects/SkArithmeticMode_gpu.h
@@ -90,11 +90,13 @@ private:
     GrXferProcessor* onCreateXferProcessor(const GrCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
+                                           bool hasMixedSamples,
                                            const DstTexture*) const override; 
 
     bool willReadDstColor(const GrCaps& caps,
                           const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const override {
+                          const GrProcOptInfo& coveragePOI,
+                          bool hasMixedSamples) const override {
         return true;
     }