switch xfermodes over to sk_sp

waiting on https://codereview.chromium.org/1835163002/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1832223002

Review URL: https://codereview.chromium.org/1832223002
diff --git a/include/effects/SkArithmeticMode.h b/include/effects/SkArithmeticMode.h
index 9e8df2e..b160dc7 100644
--- a/include/effects/SkArithmeticMode.h
+++ b/include/effects/SkArithmeticMode.h
@@ -10,8 +10,7 @@
 
 #include "SkFlattenable.h"
 #include "SkScalar.h"
-
-class SkXfermode;
+#include "SkXfermode.h"
 
 class SK_API SkArithmeticMode {
 public:
@@ -22,9 +21,15 @@
      *  k1=k3=k4=0, k2=1.0 results in returning the src
      *  k1=k2=k4=0, k3=1.0 results in returning the dst
      */
+    static sk_sp<SkXfermode> Make(SkScalar k1, SkScalar k2, SkScalar k3, SkScalar k4,
+                                  bool enforcePMColor = true);
+#ifdef SK_SUPPORT_LEGACY_XFERMODE_PTR
     static SkXfermode* Create(SkScalar k1, SkScalar k2,
                               SkScalar k3, SkScalar k4,
-                              bool enforcePMColor = true);
+                              bool enforcePMColor = true) {
+        return Make(k1, k2, k3, k4, enforcePMColor).release();
+    }
+#endif
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();