Add to GN ppapi build.

ppapi/ppapi_cpp.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.
 
 # Shared source lists between trusted and untrusted targets are stored in 
 # ppapi_sources.gypi.
 
 {
   'includes': [
     'ppapi_sources.gypi',
   ],
   'targets': [
     {
+      # GN version: //ppapi:ppapi_c
       'target_name': 'ppapi_c',
       'type': 'none',
       'all_dependent_settings': {
         'include_dirs+': [
           '..',
         ],
       },
       'sources': [
         '<@(c_source_files)',
       ],
     },
     {
+      # GN version: //ppapi:ppapi_cpp_pbjects
       'target_name': 'ppapi_cpp_objects',
       'type': 'static_library',
       'dependencies': [
         'ppapi_c'
       ],
       'include_dirs+': [
         '..',
       ],
       'sources': [
         '<@(cpp_source_files)',
@@ skipping 13 matching lines @@
           'cflags': ['-Wextra', '-pedantic'],
         }],
         ['OS=="mac"', {
           'xcode_settings': {
             'WARNING_CFLAGS': ['-Wextra', '-pedantic'],
            },
         }],
       ],
     },
     {
+      # GN version: //ppapi:ppapi_cpp
       'target_name': 'ppapi_cpp',
       'type': 'static_library',
       'dependencies': [
         'ppapi_c',
         'ppapi_cpp_objects',
       ],
       'include_dirs+': [
         '..',
       ],
       'sources': [
         'cpp/module_embedder.h',
         'cpp/ppp_entrypoints.cc',
       ],
       'conditions': [
         ['OS=="linux"', {
           'cflags': ['-Wextra', '-pedantic'],
         }],
         ['OS=="mac"', {
           'xcode_settings': {
             'WARNING_CFLAGS': ['-Wextra', '-pedantic'],
            },
         }]
       ],
     },
   ],
 }