Pepper: Add some basic tests for PPB_Audio.

ppapi/proxy/ppb_audio_proxy.cc

 // 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.
 
 #include "ppapi/proxy/ppb_audio_proxy.h"
 
 #include "base/compiler_specific.h"
 #include "base/threading/simple_thread.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/c/ppb_audio.h"
@@ skipping 121 matching lines @@
     PPB_Audio_Callback audio_callback,
     void* user_data) {
   PluginDispatcher* dispatcher = PluginDispatcher::GetForInstance(instance_id);
   if (!dispatcher)
     return 0;
 
   EnterResourceNoLock<PPB_AudioConfig_API> config(config_id, true);
   if (config.failed())
     return 0;
 
+  if (!audio_callback)
+    return 0;
+
   HostResource result;
   dispatcher->Send(new PpapiHostMsg_PPBAudio_Create(
       API_ID_PPB_AUDIO, instance_id,
       config.object()->GetSampleRate(), config.object()->GetSampleFrameCount(),
       &result));
   if (result.is_null())
     return 0;
 
   return (new Audio(result, config_id,
                     audio_callback, user_data))->GetReference();
@@ skipping 154 matching lines @@
   *foreign_shared_memory_handle = dispatcher()->ShareHandleWithRemote(
       IntToPlatformFile(shared_memory_handle), false);
   if (*foreign_shared_memory_handle == IPC::InvalidPlatformFileForTransit())
     return PP_ERROR_FAILED;
 
   return PP_OK;
 }
 
 }  // namespace proxy
 }  // namespace ppapi