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}

api/test/frame_generator_interface.cc

@@ -0,0 +1,34 @@
+/*
+ *  Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "api/test/frame_generator_interface.h"
+
+namespace webrtc {
+namespace test {
+
+// static
+const char* FrameGeneratorInterface::OutputTypeToString(
+    FrameGeneratorInterface::OutputType type) {
+  switch (type) {
+    case OutputType::kI420:
+      return "I420";
+    case OutputType::kI420A:
+      return "I420A";
+    case OutputType::kI010:
+      return "I010";
+    case OutputType::kNV12:
+      return "NV12";
+    default:
+      RTC_NOTREACHED();
+  }
+}
+
+}  // namespace test
+}  // namespace webrtc