VideoStreamDecoder skeleton.

Initial commit for the public VideoStreamDecoder. To get some initial feedback
about structuring within WebRTC this CL only contains the skeleton of the class.

Bug: webrtc:8909
Change-Id: I076bb45dd30a450b3f7ef239e69ff872dc34dcf2
Reviewed-on: https://webrtc-review.googlesource.com/62080
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22560}

api/video/video_stream_decoder_create.cc

@@ -0,0 +1,24 @@
+/*
+ *  Copyright (c) 2018 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/video/video_stream_decoder_create.h"
+
+#include "rtc_base/ptr_util.h"
+#include "video/video_stream_decoder_impl.h"
+
+namespace webrtc {
+std::unique_ptr<VideoStreamDecoder> CreateVideoStreamDecoder(
+    VideoStreamDecoder::Callbacks* callbacks,
+    VideoDecoderFactory* decoder_factory,
+    std::map<int, std::pair<SdpVideoFormat, int>> decoder_settings) {
+  return rtc::MakeUnique<VideoStreamDecoderImpl>(callbacks, decoder_factory,
+                                                 std::move(decoder_settings));
+}
+}  // namespace webrtc