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>
2 files changed