[cros] Implement WebM encoder/muxer for animated avatar capture.

media/webm/chromeos/ebml_writer.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_WEBM_CHROMEOS_EBML_WRITER_H_
+#define MEDIA_WEBM_CHROMEOS_EBML_WRITER_H_
+
+#include "base/callback.h"
+
+// This struct serves as a bridge betweeen static libmkv interface and Chrome's
+// base::Callback. Must be in the global namespace. See EbmlWriter.h.
+struct EbmlGlobal {
+  base::Callback<void(const void* buffer, unsigned long len)> write_cb;
+  base::Callback<void(const void* buffer, int buffer_size, unsigned long len)>
+      serialize_cb;
+};
+
+#endif  // MEDIA_WEBM_CHROMEOS_EBML_WRITER_H_