Add Av1 Decoder wrapper behind a build flag

Bug: webrtc:11404
Change-Id: I090ffd173d667e8845de1b986af462516b7c76e6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169452
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Michael Horowitz <mhoro@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30757}

modules/video_coding/codecs/av1/libaom_av1_decoder.h

@@ -0,0 +1,26 @@
+/*
+ *  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.
+ */
+#ifndef MODULES_VIDEO_CODING_CODECS_AV1_LIBAOM_AV1_DECODER_H_
+#define MODULES_VIDEO_CODING_CODECS_AV1_LIBAOM_AV1_DECODER_H_
+
+#include <memory>
+
+#include "absl/base/attributes.h"
+#include "api/video_codecs/video_decoder.h"
+
+namespace webrtc {
+
+ABSL_CONST_INIT extern const bool kIsLibaomAv1DecoderSupported;
+
+std::unique_ptr<VideoDecoder> CreateLibaomAv1Decoder();
+
+}  // namespace webrtc
+
+#endif  // MODULES_VIDEO_CODING_CODECS_AV1_LIBAOM_AV1_DECODER_H_