Add to GN ppapi build.

ppapi/ppapi_gl.gypi

 # 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': [
     {
+      # GN version: //ppapi:ppapi_gles2
       'target_name': 'ppapi_gles2',
       'type': 'static_library',
       'dependencies': [
         'ppapi_c',
       ],
       'include_dirs': [
         'lib/gl/include',
       ],
       'sources': [
         'lib/gl/gles2/gl2ext_ppapi.c',
         'lib/gl/gles2/gl2ext_ppapi.h',
         'lib/gl/gles2/gles2.c',
       ],
     },
   ],
 }