Fix flutter build

Flutter relies on SkCodec::kNone, which has been renamed, so define
SK_LEGACY_SKCODEC_NONE_ENUM in order to see the legacy name.

Change-Id: Icc32128c504fe988f55873b0221f56c56859def4
Reviewed-on: https://skia-review.googlesource.com/148663
Auto-Submit: Leon Scroggins <scroggo@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
diff --git a/gn/flutter_defines.gni b/gn/flutter_defines.gni
index 7875f4c..91083c7 100644
--- a/gn/flutter_defines.gni
+++ b/gn/flutter_defines.gni
@@ -6,4 +6,5 @@
   "SK_DISABLE_EXPLICIT_GPU_RESOURCE_ALLOCATION",
   "SK_DISABLE_RENDER_TARGET_SORTING",
   "SK_SUPPORT_LEGACY_VULKAN_INTERFACE",
+  "SK_LEGACY_SKCODEC_NONE_ENUM",
 ]