Add test to make sure if PPB_Audio_Shared::StartThread() works.

ppapi/proxy/ppb_audio_proxy.cc

 // 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.
 
 #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 77 matching lines @@
 
 PP_Resource Audio::GetCurrentConfig() {
   // AddRef for the caller.
   PpapiGlobals::Get()->GetResourceTracker()->AddRefResource(config_);
   return config_;
 }
 
 PP_Bool Audio::StartPlayback() {
   if (playing())
     return PP_TRUE;
-  SetStartPlaybackState();
+  if (!SetStartPlaybackState())
+    return PP_FALSE;
   PluginDispatcher::GetForResource(this)->Send(
       new PpapiHostMsg_PPBAudio_StartOrStop(
           API_ID_PPB_AUDIO, host_resource(), true));
   return PP_TRUE;
 }
 
 PP_Bool Audio::StopPlayback() {
   if (!playing())
     return PP_TRUE;
   PluginDispatcher::GetForResource(this)->Send(
@@ skipping 218 matching lines @@
     static_cast<Audio*>(enter.object())->SetStreamInfo(
         enter.resource()->pp_instance(), handle.shmem(), handle.size(),
         IPC::PlatformFileForTransitToPlatformFile(socket_handle.descriptor()),
         config.object()->GetSampleRate(),
         config.object()->GetSampleFrameCount());
   }
 }
 
 }  // namespace proxy
 }  // namespace ppapi