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

ppapi/ppapi_proxy.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.
 
 {
   'targets': [
     {
       'target_name': 'ppapi_proxy',
       'type': '<(component)',
       'dependencies': [
@@ skipping 15 matching lines @@
       'include_dirs': [
         '..',
         '../..',  # For nacl includes to work.
       ],
       'sources': [
         # Take some standalong files from the C++ wrapper allowing us to more
         # easily make async callbacks in the proxy. We can't depend on the
         # full C++ wrappers at this layer since the C++ wrappers expect
         # symbols defining the globals for "being a plugin" which we are not.
         # These callback files are standalone.
-        'cpp/completion_callback.cc',
         'cpp/completion_callback.h',
         'utility/completion_callback_factory.h',
 
         'proxy/broker_dispatcher.cc',
         'proxy/broker_dispatcher.h',
         'proxy/dispatcher.cc',
         'proxy/dispatcher.h',
         'proxy/enter_proxy.h',
         'proxy/host_dispatcher.cc',
         'proxy/host_dispatcher.h',
@@ skipping 122 matching lines @@
         'proxy/serialized_var.cc',
         'proxy/serialized_var.h',
         'proxy/var_serialization_rules.h',
       ],
       'defines': [
         'PPAPI_PROXY_IMPLEMENTATION',
       ],
     },
   ],
 }