add staging flags

Change-Id: If3d9c2b9f490d89aaf49a971271059e95d784846
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/335716
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

Author: reed-at-google

gn/flutter_defines.gni

@@ -16,6 +16,8 @@ flutter_defines = [
 
   # Staging
   "SK_SUPPORT_LEGACY_MATRIX_FACTORIES",
+  "SK_SUPPORT_LEGACY_ADJUSTHQHEURISTIC",
+  "SK_SUPPORT_LEGACY_ONDRAWIMAGERECT",
 
   # Fast low-precision software rendering isn't a priority for Flutter.
   "SK_DISABLE_LEGACY_SHADERCONTEXT",