Simplify flattening to just write enough to call the factory

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index 74fd9f85f7c3a56d80d38231a263e423ac1d46bd..3fb3ab59a79c4e47bd1b0cbc5cb891ece70b5d92 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -217,7 +217,9 @@ public:
 
 protected:
     SkXfermode() {}
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
     explicit SkXfermode(SkReadBuffer& rb) : SkFlattenable(rb) {}
+#endif
 
     /** The default implementation of xfer32/xfer16/xferA8 in turn call this
         method, 1 color at a time (upscaled to a SkPMColor). The default