Add libwebm in third_party.

third_party/libwebm/libwebm.gypi

Index: third_party/libwebm/libwebm.gypi
diff --git a/third_party/libwebm/libwebm.gypi b/third_party/libwebm/libwebm.gypi
new file mode 100644
index 0000000000000000000000000000000000000000..074110145273d07113fc3af4ea69c0e3438d958c
--- /dev/null
+++ b/third_party/libwebm/libwebm.gypi
@@ -0,0 +1,13 @@
+# Copyright 2014 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.
+
+{
+  'variables': {
+    'libwebm_sources': [
+      'source/mkvmuxer.cpp',
+      'source/mkvmuxerutil.cpp',
+      'source/mkvwriter.cpp',
+    ],
+  }
+}