Add legacy flag for vulkan interface for flutter.

Bug: skia:
Change-Id: I50eb5c331b4a5818037155865d477914276a983c
Reviewed-on: https://skia-review.googlesource.com/137194
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/gn/flutter_defines.gni b/gn/flutter_defines.gni
index d817f3c..7875f4c 100644
--- a/gn/flutter_defines.gni
+++ b/gn/flutter_defines.gni
@@ -5,4 +5,5 @@
 flutter_defines = [
   "SK_DISABLE_EXPLICIT_GPU_RESOURCE_ALLOCATION",
   "SK_DISABLE_RENDER_TARGET_SORTING",
+  "SK_SUPPORT_LEGACY_VULKAN_INTERFACE",
 ]