Use the worker task runner instead of the main audio thread.

On OSX the main audio thread is the UI thread due to limitations
in the CoreAudio API. Since we're making effectively read only
calls sending the audio hardware config from the worker task
runner should be fine. We already do this in several other places
for device enumeration (since it's way to slow on UI thread).

BUG=469737
TEST=none

Review URL: https://codereview.chromium.org/1029933003

Cr-Commit-Position: refs/heads/master@{#322292}

content/browser/renderer_host/render_process_host_impl.cc

@@ -213,7 +213,7 @@ void RemoveShaderInfo(int32 id) {
 
 scoped_ptr<IPC::Message> SendAudioHardwareConfig() {
   media::AudioManager* am = media::AudioManager::Get();
-  DCHECK(am->GetTaskRunner()->BelongsToCurrentThread());
+  DCHECK(am->GetWorkerTaskRunner()->BelongsToCurrentThread());
   return make_scoped_ptr(new ViewMsg_SetAudioHardwareConfig(
       am->GetDefaultOutputStreamParameters(),
       am->GetInputStreamParameters(media::AudioManagerBase::kDefaultDeviceId)));
@@ -2338,7 +2338,7 @@ void RenderProcessHostImpl::OnProcessLaunched() {
   }
 
   base::PostTaskAndReplyWithResult(
-      media::AudioManager::Get()->GetTaskRunner().get(), FROM_HERE,
+      media::AudioManager::Get()->GetWorkerTaskRunner().get(), FROM_HERE,
       base::Bind(&SendAudioHardwareConfig),
       base::Bind(base::IgnoreResult(&RenderProcessHostImpl::SendHelper),
                  weak_factory_.GetWeakPtr()));