Remove power monitoring from AudioThreadHangMonitor.

The current code has two bugs.
1. PowerMonitor::RemoveObserver isn't called at destruction. Oops.
2. PowerMonitor::AddObserver is called from a task scheduler
thread, which can race with the power monitor being destructed on
the main thread.

1 is straightforward to fix but 2 isn't really. The power monitor
isn't adding much value, so let's just remove it.

Bug: 912922, 912997
Change-Id: I11cbc028d6ef4a9767096f2553d1cc340aa86e57
Reviewed-on: https://chromium-review.googlesource.com/c/1371390Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615493}

media/audio/audio_thread_hang_monitor.cc

@@ -8,7 +8,6 @@
 #include <utility>
 
 #include "base/bind.h"
-#include "base/debug/alias.h"
 #include "base/debug/dump_without_crashing.h"
 #include "base/location.h"
 #include "base/metrics/histogram_macros.h"
@@ -80,12 +79,6 @@ AudioThreadHangMonitor::AudioThreadHangMonitor(
 void AudioThreadHangMonitor::StartTimer() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(monitor_sequence_);
 
-  auto* pm = base::PowerMonitor::Get();
-  if (pm) {
-    // May be null in unit tests.
-    pm->AddObserver(this);
-  }
-
   // Set the flag to true so that the first run doesn't detect a hang.
   alive_flag_->flag_ = true;
 
@@ -110,12 +103,6 @@ bool AudioThreadHangMonitor::NeverLoggedThreadRecoveredAfterHung() const {
   return audio_thread_status_ == ThreadStatus::kHung;
 }
 
-void AudioThreadHangMonitor::OnResume() {
-  DCHECK_CALLED_ON_VALID_SEQUENCE(monitor_sequence_);
-  last_resume_time_ = clock_->NowTicks();
-  recent_ping_state_ = 0;
-}
-
 void AudioThreadHangMonitor::CheckIfAudioThreadIsAlive() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(monitor_sequence_);
 
@@ -152,9 +139,6 @@ void AudioThreadHangMonitor::CheckIfAudioThreadIsAlive() {
       LogHistogramThreadStatus();
 
       if (dump_on_hang_) {
-        int64_t time_since_resume =
-            (clock_->NowTicks() - last_resume_time_).InMilliseconds();
-        base::debug::Alias(&time_since_resume);
         base::debug::DumpWithoutCrashing();
       }
     }

media/audio/audio_thread_hang_monitor.h

@@ -13,7 +13,6 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_refptr.h"
-#include "base/power_monitor/power_observer.h"
 #include "base/sequence_checker.h"
 #include "base/sequenced_task_runner.h"
 #include "base/time/time.h"
@@ -34,7 +33,7 @@ namespace media {
 // posting a task to the audio thread every minute and checking that it was
 // executed. If three consecutive such pings are missed, the thread is
 // considered hung.
-class MEDIA_EXPORT AudioThreadHangMonitor final : public base::PowerObserver {
+class MEDIA_EXPORT AudioThreadHangMonitor final {
  public:
   using Ptr =
       std::unique_ptr<AudioThreadHangMonitor, base::OnTaskRunnerDeleter>;
@@ -57,7 +56,7 @@ class MEDIA_EXPORT AudioThreadHangMonitor final : public base::PowerObserver {
       scoped_refptr<base::SingleThreadTaskRunner> audio_thread_task_runner,
       scoped_refptr<base::SequencedTaskRunner> monitor_task_runner = nullptr);
 
-  ~AudioThreadHangMonitor() final;
+  ~AudioThreadHangMonitor();
 
   // Thread-safe.
   bool IsAudioThreadHung() const;
@@ -85,10 +84,6 @@ class MEDIA_EXPORT AudioThreadHangMonitor final : public base::PowerObserver {
   bool NeverLoggedThreadHung() const;
   bool NeverLoggedThreadRecoveredAfterHung() const;
 
-  // base::PowerObserver overrides.
-  // Reset hang detection state when the system comes out of the suspend state.
-  void OnResume() final;
-
   // This function is run by the |timer_|. It checks if the audio thread has
   // shown signs of life since the last time it was called (by checking the
   // |alive_flag_|) and updates the value of |successful_pings_| and
@@ -126,9 +121,6 @@ class MEDIA_EXPORT AudioThreadHangMonitor final : public base::PowerObserver {
   // detecting the audio thread as hung.
   base::TimeTicks last_check_time_ = base::TimeTicks();
 
-  // Used to investigate hangs.
-  base::TimeTicks last_resume_time_ = base::TimeTicks();
-
   // |recent_ping_state_| tracks the recent life signs from the audio thread. If
   // the most recent ping was successful, the number indicates the number of
   // successive successful pings. If the most recent ping was failed, the number