add flag to stage change to shader factories

Bug: skia:8937
Change-Id: I59be04380805a78b7a9e24721e4f4349797735f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1558685
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649254}

skia/config/SkUserConfig.h

@@ -154,6 +154,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_DISABLE_OPLIST_SORTING
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_SHADER_FACTORIES
+#define SK_SUPPORT_LEGACY_SHADER_FACTORIES
+#endif
+
 #ifndef SK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS
 #define SK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS
 #endif