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

chrome/browser/chromeos/ebml_writer.cc

+// 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.
+
+#include "chrome/browser/chromeos/ebml_writer.h"
+
+extern "C" {
+#include "third_party/libvpx/source/libvpx/libmkv/EbmlWriter.h"
+
+// These functions must be in the global namespace and visible to libmkv.
+
+__attribute__((__visibility__("default")))
+void Ebml_Write(EbmlGlobal *glob, const void *buffer, unsigned long len) {
+  glob->write_cb.Run(buffer, len);
+}
+
+__attribute__((__visibility__("default")))
+void Ebml_Serialize(EbmlGlobal *glob,
+                    const void *buffer,
+                    int buffer_size,
+                    unsigned long len) {
+  glob->serialize_cb.Run(buffer, buffer_size, len);
+}
+
+}  // extern "C"