add flag for future Skia CL

Future CL is here: https://skia-review.googlesource.com/c/skia/+/87788

Bug: 
Change-Id: Ib0c4988bdbbaf076f91b1f37f51ddc50d12b57c5
Reviewed-on: https://chromium-review.googlesource.com/836930Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525360}

skia/config/SkUserConfig.h

@@ -241,6 +241,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_DASH_CULL_PATH
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_SERIALPROCS_REF
+#define SK_SUPPORT_LEGACY_SERIALPROCS_REF
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,