Add support for NV12 frame generation for tests

This can be used in the future to test NV12 video frames with encoders, both
from unittests and from tools like video_loopback.

Tested using video_loopback with generator NV12.

Bug: webrtc:11978
Change-Id: I0d24ae3ebab2267f076703cbda81e99cec465ec8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185045
Commit-Queue: Evan Shrubsole <eshr@google.com>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32206}
diff --git a/api/test/frame_generator_interface.h b/api/test/frame_generator_interface.h
index 691b6ee..6d8da3d 100644
--- a/api/test/frame_generator_interface.h
+++ b/api/test/frame_generator_interface.h
@@ -32,7 +32,7 @@
     absl::optional<VideoFrame::UpdateRect> update_rect;
   };
 
-  enum class OutputType { kI420, kI420A, kI010 };
+  enum class OutputType { kI420, kI420A, kI010, kNV12 };
 
   virtual ~FrameGeneratorInterface() = default;