Let target egl_main_native build without error.

gpu/gles2_conform_support/gles2_conform_support.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': {
     # These are defined here because we want to be able to compile them on
     # the buildbots without needed the OpenGL ES 2.0 conformance tests
     # which are not open source.
     'bootstrap_sources_native': [
@@ skipping 56 matching lines @@
         }],
       ],
       'include_dirs': ['<(DEPTH)/third_party/khronos'],
       'sources': [
         '<@(bootstrap_sources_native)',
       ],
       'direct_dependent_settings': {
         'include_dirs': ['<(DEPTH)/third_party/khronos'],
       },
       'defines': [
+        'GLES2_CONFORM_SUPPORT_ONLY',
         'GTF_GLES20',
         'EGLAPI=',
         'EGLAPIENTRY=',
       ],
     },
     {
       'target_name': 'gles2_conform_support',
       'type': 'executable',
       'dependencies': [
         'egl_native',
@@ skipping 12 matching lines @@
         'EGLAPI=',
         'EGLAPIENTRY=',
       ],
       'sources': [
         '<@(bootstrap_sources_native)',
         'gles2_conform_support.c'
       ],
     },
   ],
 }