Add placeholder header file api/video_codecs/video_codec.h.

This is a preparation for moving declaration of the VideoCodec class
to this file. Applications using the legacy VideoCodec class should
start including this file instead of common_types.h.

Bug: webrtc:7660
Change-Id: I9fe1a2bffa7b0c17059fc4e3b7e351f5019f1d00
Reviewed-on: https://webrtc-review.googlesource.com/80150
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23465}
diff --git a/api/video_codecs/video_codec.h b/api/video_codecs/video_codec.h
new file mode 100644
index 0000000..081a612
--- /dev/null
+++ b/api/video_codecs/video_codec.h
@@ -0,0 +1,23 @@
+/*
+ *  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.
+ */
+
+#ifndef API_VIDEO_CODECS_VIDEO_CODEC_H_
+#define API_VIDEO_CODECS_VIDEO_CODEC_H_
+
+// TODO(bugs.webrtc.org/7660): This is an initial place holder file. Downstream
+// users of VideoCodec must be updated to include this file, before contents can
+// be moved out of common_types.h.
+
+#include "common_types.h"  // NOLINT(build/include)
+
+// The VideoCodec class represents an old defacto-api, which we're migrating
+// away from slowly.
+
+#endif  // API_VIDEO_CODECS_VIDEO_CODEC_H_