Accept NV12 frames into VP9

NV12 frames can be encoded by libvpx now, and this change allows for
encoding of them with VP9.

VP9 encode/decode tests now run with NV12 as well as I420.

Manually tested using video loopback with VP9 and NV12 generated frames.
  out/Default/video_loopback.app/Contents/MacOS/video_loopback --clip=GeneratorNV12 --codec="VP9"


Bug: webrtc:11635, webrtc:11974
Change-Id: Ifc5cbf77d2a27821cd5560c253d5d447c7a7cf53
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185123
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/master@{#32220}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -98,6 +98,8 @@ class VP9EncoderImpl : public VP9Encoder {
 
   size_t SteadyStateSize(int sid, int tid);
 
+  void MaybeRewrapRawWithFormat(const vpx_img_fmt fmt);
+
   EncodedImage encoded_image_;
   CodecSpecificInfo codec_specific_;
   EncodedImageCallback* encoded_complete_callback_;