add flag for flutter

Bug: skia:
Change-Id: I707909dbd43b8530ecf3697da78cf567d76d71e9
Reviewed-on: https://skia-review.googlesource.com/c/183385
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/gn/flutter_defines.gni b/gn/flutter_defines.gni
index bd0b95d..7e582d3 100644
--- a/gn/flutter_defines.gni
+++ b/gn/flutter_defines.gni
@@ -14,6 +14,7 @@
   "SK_DISABLE_DAA",
 
   # API staging
+  "SK_SUPPORT_LEGACY_PAINT_FONT_FIELDS",
 
   # Flutter doesn't deserialize anything.
   "SK_DISABLE_READBUFFER",