Add Arithmetic mode xp.

src/effects/SkArithmeticMode.cpp

Index: src/effects/SkArithmeticMode.cpp
diff --git a/src/effects/SkArithmeticMode.cpp b/src/effects/SkArithmeticMode.cpp
index d86f73bee45b1510737ecd08614c7c2daeed50f6..ae18c2770e75e39e719a5aa7a78b2a38f990368f 100644
--- a/src/effects/SkArithmeticMode.cpp
+++ b/src/effects/SkArithmeticMode.cpp
@@ -31,8 +31,9 @@ public:
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkArithmeticMode_scalar)
 
 #if SK_SUPPORT_GPU
-    virtual bool asFragmentProcessor(GrFragmentProcessor**,
-                                     GrTexture* background) const SK_OVERRIDE;
+    bool asFragmentProcessor(GrFragmentProcessor**, GrTexture* background) const SK_OVERRIDE;
+
+    bool asXPFactory(GrXPFactory**) const SK_OVERRIDE;
 #endif
 
 private:
@@ -246,6 +247,17 @@ bool SkArithmeticMode_scalar::asFragmentProcessor(GrFragmentProcessor** fp,
     return true;
 }
 
+bool SkArithmeticMode_scalar::asXPFactory(GrXPFactory** xpf) const {
+    if (xpf) {
+        *xpf = GrArithmeticXPFactory::Create(SkScalarToFloat(fK[0]),
+                                             SkScalarToFloat(fK[1]),
+                                             SkScalarToFloat(fK[2]),
+                                             SkScalarToFloat(fK[3]),
+                                             fEnforcePMColor);
+    }
+    return true;
+}
+
 #endif
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkArithmeticMode)