Revert of Remove SK_SUPPORT_LEGACY_DEEPFLATTENING.

src/opts/SkXfermode_opts_arm_neon.h

Index: src/opts/SkXfermode_opts_arm_neon.h
diff --git a/src/opts/SkXfermode_opts_arm_neon.h b/src/opts/SkXfermode_opts_arm_neon.h
index f091432d88ebfef19e1ea4189f435e928719f8c9..745bfe260ce42c6fb99cf21066886c0651271754 100644
--- a/src/opts/SkXfermode_opts_arm_neon.h
+++ b/src/opts/SkXfermode_opts_arm_neon.h
@@ -17,6 +17,10 @@
     SK_TO_STRING_OVERRIDE()
 
 private:
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+    SkNEONProcCoeffXfermode(SkReadBuffer& buffer);
+#endif
+
     // void* is used to avoid pulling arm_neon.h in the core and having to build
     // it with -mfpu=neon.
     void* fProcSIMD;