Add libwebm in third_party.

third_party/libwebm/libwebm_nacl.gyp

 # 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',
     '../../native_client/build/untrusted.gypi',
   ],
   'targets': [
     {
-      'target_name': 'modp_b64_nacl',
+      'target_name': 'libwebm_nacl',
       'type': 'none',
       'variables': {
-        'nlib_target': 'libmodp_b64_nacl.a',
+        'nlib_target': 'libwebm_nacl.a',
         'build_glibc': 0,
-        'build_newlib': 1,
+        'build_newlib': 0,
         'build_pnacl_newlib': 1,
       },
       'dependencies': [
-        '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
+        '../../native_client/tools.gyp:prep_toolchain',
       ],
       'sources': [
-        'modp_b64.cc',
-        'modp_b64.h',
-        'modp_b64_data.h',
+        '<@(libwebm_sources)',
       ],
-    },
-  ],
+    },  # target libwebm_nacl
+  ]
 }