remove libwebm third_party dependency

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': 'libwebm_nacl',
-      'type': 'none',
-      'variables': {
-        'nlib_target': 'libwebm_nacl.a',
-        'build_glibc': 0,
-        'build_newlib': 0,
-        'build_pnacl_newlib': 1,
-      },
-      'dependencies': [
-        '../../native_client/tools.gyp:prep_toolchain',
-      ],
-      'sources': [
-        '<@(libwebm_sources)',
-      ],
-      'defines': [
-        # LLONG_MIN and UULONG_MAX are defined in newlib only for C++11.
-        'LLONG_MIN=LONG_LONG_MIN',
-        'ULLONG_MAX=ULONG_LONG_MAX',
-      ],
-    },  # target libwebm_nacl
-  ]
-}