Revert of Add lock to fix race in AudioRendererMixerInput. (patchset #5 id:80001 of https://codereview.chromium.org/1748183006/ )

Reason for revert:
This seems to cause failures in LayoutTests/http/tests/security/media. Not sure why this wasn't caught by CQ...

Tracking CQ weirdness here:
https://bugs.chromium.org/p/chromium/issues/detail?id=592079

Original issue's description:
> Add lock to fix race in AudioRendererMixerInput.
>
> Clusterfuzz identified a race between the media thread calling SetVolume
> and the audio device thread calling ProvideInput. Add a lock to
> synchronize access to |volume_| between threads.
>
> Also adds some thread_checkers to just firm up the contract that the
> majority of these methods are called only by the media thread.
>
> BUG=588992
>
> Committed: https://crrev.com/57c6958a99f2c62a4408d2a9d262a22129309106
> Cr-Commit-Position: refs/heads/master@{#379349}

TBR=dalecurtis@chromium.org,olka@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=588992

Review URL: https://codereview.chromium.org/1771463002

Cr-Commit-Position: refs/heads/master@{#379388}

2 files changed, 2 insertions, 43 deletions

diff --git a/media/base/audio_renderer_mixer_input.cc b/media/base/audio_renderer_mixer_input.cc
index f990538..5743700 100644
--- a/media/base/audio_renderer_mixer_input.cc
+++ b/media/base/audio_renderer_mixer_input.cc
@@ -29,22 +29,15 @@ AudioRendererMixerInput::AudioRendererMixerInput(
       mixer_(nullptr),
       callback_(nullptr),
       error_cb_(base::Bind(&AudioRendererMixerInput::OnRenderError,
-                           base::Unretained(this))) {
-  // Can be constructed on any thread, but sink operations should all occur
-  // on same thread.
-  thread_checker_.DetachFromThread();
-}
+                           base::Unretained(this))) {}
 
 AudioRendererMixerInput::~AudioRendererMixerInput() {
-  DCHECK(!started_);
   DCHECK(!mixer_);
 }
 
 void AudioRendererMixerInput::Initialize(
     const AudioParameters& params,
     AudioRendererSink::RenderCallback* callback) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  DCHECK(!started_);
   DCHECK(!mixer_);
   DCHECK(callback);
 
@@ -53,7 +46,6 @@ void AudioRendererMixerInput::Initialize(
 }
 
 void AudioRendererMixerInput::Start() {
-  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(!started_);
   DCHECK(!mixer_);
   DCHECK(callback_);  // Initialized.
@@ -76,8 +68,6 @@ void AudioRendererMixerInput::Start() {
 }
 
 void AudioRendererMixerInput::Stop() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   // Stop() may be called at any time, if Pause() hasn't been called we need to
   // remove our mixer input before shutdown.
   Pause();
@@ -100,8 +90,6 @@ void AudioRendererMixerInput::Stop() {
 }
 
 void AudioRendererMixerInput::Play() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   if (playing_ || !mixer_)
     return;
 
@@ -110,8 +98,6 @@ void AudioRendererMixerInput::Play() {
 }
 
 void AudioRendererMixerInput::Pause() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   if (!playing_ || !mixer_)
     return;
 
@@ -120,8 +106,6 @@ void AudioRendererMixerInput::Pause() {
 }
 
 bool AudioRendererMixerInput::SetVolume(double volume) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  base::AutoLock auto_lock(volume_lock_);
   volume_ = volume;
   return true;
 }
@@ -134,8 +118,6 @@ void AudioRendererMixerInput::SwitchOutputDevice(
     const std::string& device_id,
     const url::Origin& security_origin,
     const SwitchOutputDeviceCB& callback) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   if (!mixer_) {
     if (pending_switch_callback_.is_null()) {
       pending_switch_callback_ = callback;
@@ -177,16 +159,12 @@ void AudioRendererMixerInput::SwitchOutputDevice(
 }
 
 AudioParameters AudioRendererMixerInput::GetOutputParameters() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   if (mixer_)
     return mixer_->GetOutputDevice()->GetOutputParameters();
   return get_hardware_params_cb_.Run(device_id_, security_origin_);
 }
 
 OutputDeviceStatus AudioRendererMixerInput::GetDeviceStatus() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-
   if (mixer_)
     return mixer_->GetOutputDevice()->GetDeviceStatus();
 
@@ -198,11 +176,6 @@ OutputDeviceStatus AudioRendererMixerInput::GetDeviceStatus() {
 
 double AudioRendererMixerInput::ProvideInput(AudioBus* audio_bus,
                                              base::TimeDelta buffer_delay) {
-  // No thread checker here. This method is called on a different thread as part
-  // of audio rendering. AudioRendererMixer has locks that protect us from
-  // things like attempting to ProvideInput while simultaneously removing
-  // ourselves from mixer inputs (see Pause()).
-
   // TODO(chcunningham): Delete this conversion and change ProvideInput to more
   // precisely describe delay as a count of frames delayed instead of TimeDelta.
   // See http://crbug.com/587522.
@@ -217,11 +190,7 @@ double AudioRendererMixerInput::ProvideInput(AudioBus* audio_bus,
         frames_filled, audio_bus->frames() - frames_filled);
   }
 
-  // Synchronize access to |volume_| with SetVolume().
-  {
-    base::AutoLock auto_lock(volume_lock_);
-    return frames_filled > 0 ? volume_ : 0;
-  }
+  return frames_filled > 0 ? volume_ : 0;
 }
 
 void AudioRendererMixerInput::OnRenderError() {
diff --git a/media/base/audio_renderer_mixer_input.h b/media/base/audio_renderer_mixer_input.h
index 6ae5a4bc..34f37c5 100644
--- a/media/base/audio_renderer_mixer_input.h
+++ b/media/base/audio_renderer_mixer_input.h
@@ -9,8 +9,6 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
-#include "base/synchronization/lock.h"
-#include "base/threading/thread_checker.h"
 #include "media/base/audio_converter.h"
 #include "media/base/audio_renderer_sink.h"
 #include "media/base/output_device.h"
@@ -71,14 +69,6 @@ class MEDIA_EXPORT AudioRendererMixerInput
  private:
   friend class AudioRendererMixerInputTest;
 
-  // Used to DCHECK that control methods (Start/Stop/Switch...) are called from
-  // the same thread.
-  base::ThreadChecker thread_checker_;
-
-  // Protect |volume_|, accessed by separate threads in ProvideInput() and
-  // SetVolume().
-  base::Lock volume_lock_;
-
   bool started_;
   bool playing_;
   double volume_;