Fix all relative includes to third_party.

samples/samples.gyp

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'pdf_enable_v8%': 1,
   },
   'target_defaults': {
     'defines' : [
       'PNG_PREFIX',
       'PNGPREFIX_H',
       'PNG_USE_READ_MACROS',
     ],
     'include_dirs': [
+      # This is implicit in GN.
       '<(DEPTH)',
+      '..',
     ],
     'conditions': [
       ['pdf_enable_v8==1', {
         'defines': [
           'PDF_ENABLE_V8',
         ],
         'include_dirs': [
           '<(DEPTH)/v8',
           '<(DEPTH)/v8/include',
         ],
@@ skipping 32 matching lines @@
     },
     {
       'target_name': 'pdfium_diff',
       'type': 'executable',
       'variables': { 'enable_wexit_time_destructors': 1, },
       'dependencies': [
         'fx_lpng',
         '../pdfium.gyp:pdfium',
         '../third_party/third_party.gyp:pdfium_base',
       ],
-      'include_dirs': [
-        '../../',
-      ],
       'sources': [
         'image_diff.cc',
         'image_diff_png.h',
         'image_diff_png.cc',
       ],
     },
     {
       'target_name': 'fx_lpng',
       'type': 'static_library',
       'dependencies': [
@@ skipping 16 matching lines @@
         'fx_lpng/src/fx_pngset.c',
         'fx_lpng/src/fx_pngtrans.c',
         'fx_lpng/src/fx_pngwio.c',
         'fx_lpng/src/fx_pngwrite.c',
         'fx_lpng/src/fx_pngwtran.c',
         'fx_lpng/src/fx_pngwutil.c',
       ],
     },
   ],
 }