Add Arithmetic mode xp.

src/gpu/GrOptDrawState.cpp

Index: src/gpu/GrOptDrawState.cpp
diff --git a/src/gpu/GrOptDrawState.cpp b/src/gpu/GrOptDrawState.cpp
index f9febf445b87bd6afdc9ab23718738af5760f1ff..f1e2b189f38e6b2df4ade97183cd2f813cddc907 100644
--- a/src/gpu/GrOptDrawState.cpp
+++ b/src/gpu/GrOptDrawState.cpp
@@ -155,6 +155,10 @@ void GrOptDrawState::adjustProgramFromOptimizations(const GrDrawState& ds,
             fDescInfo.fReadsFragPosition = true;
         }
     }
+
+    if (fXferProcessor->willReadDstColor()) {
+        fDescInfo.fReadsDst = true;
+    }
 }
 
 void GrOptDrawState::finalize(GrGpu* gpu) {