Revert of Remove SK_SUPPORT_LEGACY_DEEPFLATTENING.

src/effects/SkLerpXfermode.cpp

Index: src/effects/SkLerpXfermode.cpp
diff --git a/src/effects/SkLerpXfermode.cpp b/src/effects/SkLerpXfermode.cpp
index 76c97ef9acbd0e7086f9c3f0766828fa48b82259..0376a57b64f0a2921d707123a11af1883413ab00 100644
--- a/src/effects/SkLerpXfermode.cpp
+++ b/src/effects/SkLerpXfermode.cpp
@@ -22,6 +22,12 @@
 }
 
 SkLerpXfermode::SkLerpXfermode(unsigned scale256) : fScale256(scale256) {}
+
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+SkLerpXfermode::SkLerpXfermode(SkReadBuffer& buffer) : INHERITED(buffer) {
+    fScale256 = buffer.readUInt();
+}
+#endif
 
 void SkLerpXfermode::flatten(SkWriteBuffer& buffer) const {
     buffer.writeUInt(fScale256);