Add GN arg to allow Flutter-specific API guards

Bug: skia:
Change-Id: I5a97da664626eebf30bba3f391fff0d8b9e62ea5
Reviewed-on: https://skia-review.googlesource.com/23042
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
diff --git a/gn/flutter_defines.gni b/gn/flutter_defines.gni
new file mode 100644
index 0000000..75ac43d
--- /dev/null
+++ b/gn/flutter_defines.gni
@@ -0,0 +1,5 @@
+# Copyright 2017 Google Inc.
+#
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+flutter_defines = [ "SK_SUPPORT_LEGACY_IMAGE_ENCODE_API" ]