Generic Frame Descriptor (GFD) VP8 templates.

In this CL:
 - Updated Vp8TemporalLayers::OnEncodeDone to take a CodecSpecificInfo
   instead of a CodecSpecificInfoVP8, so that both the VP8 specific and
   generic information can be populated.
 - Added structs to represent the GFD template structure.
 - Added code to generate templates for video/screensharing.

Bug: webrtc:10342
Change-Id: I978f9d708597a6f86bbdc494e62acf7a7b400db3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/123422
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26987}
diff --git a/modules/video_coding/include/video_codec_interface.cc b/modules/video_coding/include/video_codec_interface.cc
new file mode 100644
index 0000000..bd033b6
--- /dev/null
+++ b/modules/video_coding/include/video_codec_interface.cc
@@ -0,0 +1,20 @@
+/*
+ *  Copyright (c) 2019 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 "modules/video_coding/include/video_codec_interface.h"
+
+namespace webrtc {
+CodecSpecificInfo::CodecSpecificInfo() : codecType(kVideoCodecGeneric) {
+  memset(&codecSpecific, 0, sizeof(codecSpecific));
+}
+
+CodecSpecificInfo::CodecSpecificInfo(const CodecSpecificInfo&) = default;
+CodecSpecificInfo::~CodecSpecificInfo() = default;
+
+}  // namespace webrtc