PPAPI: Add C++ wrapper for MessageHandler stuff.

ppapi/ppapi_sources.gypi

 # 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': {
     'c_source_files': [
       'c/pp_array_output.h',
       'c/pp_bool.h',
       'c/pp_codecs.h',
@@ skipping 179 matching lines @@
       'cpp/input_event.h',
       'cpp/instance.cc',
       'cpp/instance.h',
       'cpp/instance_handle.cc',
       'cpp/instance_handle.h',
       'cpp/logging.h',
       'cpp/media_stream_audio_track.cc',
       'cpp/media_stream_audio_track.h',
       'cpp/media_stream_video_track.cc',
       'cpp/media_stream_video_track.h',
+      'cpp/message_handler.h',
       'cpp/message_loop.cc',
       'cpp/message_loop.h',
       'cpp/module.cc',
       'cpp/module.h',
       'cpp/module_impl.h',
       'cpp/mouse_cursor.cc',
       'cpp/mouse_cursor.h',
       'cpp/mouse_lock.cc',
       'cpp/mouse_lock.h',
       'cpp/net_address.cc',
@@ skipping 364 matching lines @@
       'tests/test_x509_certificate_private.h',
 
       # Deprecated test cases.
       'tests/test_instance_deprecated.cc',
       'tests/test_instance_deprecated.h',
       'tests/test_var_deprecated.cc',
       'tests/test_var_deprecated.h',
     ],
   },
 }