Fixing deadlock by removing unnesessary lock in AudioRendererMixer

Deadlock is like this:

On main or media thread:
ARMM::GetOutputDeviceInfo [takes |lock_|]
-> AOD::GetOutputDeviceInfo -> waits for device authorization, which can be signalled on IO thread only

On IO thread:
AOD::CreateStreamOnIOThread/OnStateChanged*
-> ARMM::OnRenderError [takes |lock_|],
so OnDeviceAuthorized/OnIPCClosed will never be called and device authorization will never be signalled.

*AOD::OnDeviceAuthorized itself does not deadlock it always signals event before calling OnRenderError()

That lock in ARMM::GetOutputDeviceInfo is never needed, after all

BUG=615589

Review-Url: https://codereview.chromium.org/2085603002
Cr-Commit-Position: refs/heads/master@{#400735}

media/base/audio_renderer_mixer.cc

@@ -115,7 +115,6 @@ void AudioRendererMixer::RemoveErrorCallback(const base::Closure& error_cb) {
 
 OutputDeviceInfo AudioRendererMixer::GetOutputDeviceInfo() {
   DVLOG(1) << __FUNCTION__;
-  base::AutoLock auto_lock(lock_);
   return audio_sink_->GetOutputDeviceInfo();
 }