Make checkdeps.py check third_party.

third_party/mach_override/mach_override.gyp

 # Copyright (c) 2011 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.
 
 {
   'targets': [
   ],
   'conditions': [
     ['OS=="mac" or (OS=="ios" and "<(GENERATOR)"=="ninja")', {
       'targets' : [
@@ skipping 42 matching lines @@
           'target_name' : 'mach_override',
           'type': 'static_library',
           'toolsets': ['host', 'target'],
           'dependencies': [
             'libudis86',
           ],
           'sources': [
             'mach_override.c',
             'mach_override.h',
           ],
+          'include_dirs': [
+            '../..',
+          ],
         },
       ],
     }],
   ],
 }