Define SK_GL in Flutter.

Change-Id: Ia4e035be40b712758821727add1fa730b95362c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215828
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed