Simplify flattening to just write enough to call the factory

src/effects/SkPixelXorXfermode.cpp

Index: src/effects/SkPixelXorXfermode.cpp
diff --git a/src/effects/SkPixelXorXfermode.cpp b/src/effects/SkPixelXorXfermode.cpp
index 129f1823d5fc4c6e1220f0c347ba1339fd4c3ca8..68b5306a5262cf1e6872dcecb9bc15ffd5ea69fa 100644
--- a/src/effects/SkPixelXorXfermode.cpp
+++ b/src/effects/SkPixelXorXfermode.cpp
@@ -22,14 +22,18 @@ SkPMColor SkPixelXorXfermode::xferColor(SkPMColor src, SkPMColor dst) const {
 }
 
 void SkPixelXorXfermode::flatten(SkWriteBuffer& wb) const {
-    this->INHERITED::flatten(wb);
     wb.writeColor(fOpColor);
 }
 
-SkPixelXorXfermode::SkPixelXorXfermode(SkReadBuffer& rb)
-        : INHERITED(rb) {
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+SkPixelXorXfermode::SkPixelXorXfermode(SkReadBuffer& rb) : INHERITED(rb) {
     fOpColor = rb.readColor();
 }
+#endif
+
+SkFlattenable* SkPixelXorXfermode::CreateProc(SkReadBuffer& buffer) {
+    return Create(buffer.readColor());
+}
 
 #ifndef SK_IGNORE_TO_STRING
 void SkPixelXorXfermode::toString(SkString* str) const {