Make checkdeps.py check third_party.

third_party/jsoncpp/jsoncpp.gypi

 # Copyright 2013 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.
 
 {
   'target_defaults': {
     'defines': [
       'JSON_USE_EXCEPTION=0',
     ],
     'sources': [
       'source/include/json/assertions.h',
       'source/include/json/autolink.h',
       'source/include/json/config.h',
       'source/include/json/features.h',
       'source/include/json/forwards.h',
       'source/include/json/json.h',
       'source/include/json/reader.h',
       'overrides/include/json/value.h',
       'source/include/json/writer.h',
       'source/src/lib_json/json_batchallocator.h',
       'overrides/src/lib_json/json_reader.cpp',
       'source/src/lib_json/json_tool.h',
       'overrides/src/lib_json/json_value.cpp',
       'source/src/lib_json/json_writer.cpp',
     ],
     'include_dirs': [
+      '../..',
       'overrides/include/',
       'source/include/',
       'source/src/lib_json/',
     ],
     'direct_dependent_settings': {
       'include_dirs': [
         'overrides/include/',
         'source/include/',
       ],
     },
   },
 }