Linux: use generated shim headers for system mesa.

third_party/khronos/khronos.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'use_system_khronos%': 0,
   },
   'conditions': [
     ['use_system_khronos==0', {
       'targets': [
         {
           'target_name': 'khronos_headers',
           'type': 'none',
           'all_dependent_settings': {
             'include_dirs': [
               '.',
               '../../gpu',  # Contains GLES2/gl2chromium.h
             ],
           },
         },
       ],
     }, { # use_system_khronos==1
       'targets': [
         {
           'target_name': 'khronos_headers',
           'type': 'none',
+          'variables': {
+            'headers_root_path': '../..',
+            'header_filenames': [
+              'GLES2/gl2.h;<GLES2/gl2chromium.h>;',
+              'GLES2/gl2ext.h;<GLES2/gl2chromium.h>;<GLES2/gl2extchromium.h>',
+            ],
+            'shim_generator_additional_args': [
+              '--headers-root', '.',
+              '--use-include-next',
+            ],
+          },
+          'includes': [
+            '../../build/shim_headers.gypi',
+          ],
           'all_dependent_settings': {
             'include_dirs': [
               '../../gpu',  # Contains GLES2/gl2chromium.h
             ],
           },
         },
       ],
     }],
   ],
 }