Add libwebm in third_party.

third_party/libwebm/libwebm.gyp

Index: third_party/libwebm/libwebm.gyp
diff --git a/third_party/libwebm/libwebm.gyp b/third_party/libwebm/libwebm.gyp
new file mode 100644
index 0000000000000000000000000000000000000000..e27740765db46ceed6b30822f920ec10c79118d6
--- /dev/null
+++ b/third_party/libwebm/libwebm.gyp
@@ -0,0 +1,18 @@
+# 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.
+
+{
+  'includes': [
+    'libwebm.gypi',
+  ],
+  'targets': [
+    {
+      'target_name': 'libwebm',
+      'type': 'static_library',
+      'sources': [
+        '<@(libwebm_sources)'
+      ]
+    },  # target libwebm
+  ]
+}