switch xfermodes over to sk_sp

src/core/SkReadBuffer.h

Index: src/core/SkReadBuffer.h
diff --git a/src/core/SkReadBuffer.h b/src/core/SkReadBuffer.h
index 46f6b818daf5b6149308c697d82cdbc9e6b9a762..960f2a36878c2b4181191788e3f83222d701fb99 100644
--- a/src/core/SkReadBuffer.h
+++ b/src/core/SkReadBuffer.h
@@ -145,7 +145,9 @@ public:
         return sk_sp<SkRasterizer>(this->readFlattenable<SkRasterizer>());
     }
     sk_sp<SkShader> readShader()     { return sk_sp<SkShader>(this->readFlattenable<SkShader>()); }
-    SkXfermode*    readXfermode()    { return this->readFlattenable<SkXfermode>(); }
+    sk_sp<SkXfermode> readXfermode() {
+        return sk_sp<SkXfermode>(this->readFlattenable<SkXfermode>());
+    }
 
     /**
      *  Like readFlattenable() but explicitly just skips the data that was written for the