Work around linker error of multiple definitions of pp::BlockUntilComplete

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_completion_callback.h',
@@ skipping 118 matching lines @@
       'c/trusted/ppb_url_loader_trusted.h',
       'c/trusted/ppp_broker.h',
     ],
     'cpp_source_files': [
       'cpp/array_output.cc',
       'cpp/array_output.h',
       'cpp/audio.cc',
       'cpp/audio.h',
       'cpp/audio_config.cc',
       'cpp/audio_config.h',
-      'cpp/completion_callback.cc',
       'cpp/completion_callback.h',
       'cpp/core.cc',
       'cpp/core.h',
       'cpp/file_io.cc',
       'cpp/file_io.h',
       'cpp/file_ref.cc',
       'cpp/file_ref.h',
       'cpp/file_system.cc',
       'cpp/file_system.h',
       'cpp/fullscreen.cc',
@@ skipping 299 matching lines @@
           'c/dev/ppb_transport_dev.h',
         ],
         'cpp_source_files': [
           'cpp/dev/transport_dev.cc',
           'cpp/dev/transport_dev.h',
         ],
       },
     }],
   ],
 }