Add flag to stage change to picture-shader

Change-Id: I3a0f849b89dfed896a64c6774cb1acabfab65425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737415
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
Cr-Commit-Position: refs/heads/master@{#859866}

skia/config/SkUserConfig.h

@@ -204,6 +204,8 @@ SK_API void SkDebugf_FileLine(const char* file,
 
 #define SK_SUPPORT_LEGACY_MATRIX_IMAGEFILTER
 
+#define SK_SUPPORT_LEGACY_PICTURESHADER_MATH
+
 // For now, Chrome should only attempt to reduce opList splitting when recording
 // DDLs
 #define SK_DISABLE_REDUCE_OPLIST_SPLITTING