Move OomKillsMonitor from a single-threaded SequencedWorkerPool to a non-joinable thread.

components/arc/metrics/oom_kills_monitor.h

Index: components/arc/metrics/oom_kills_monitor.h
diff --git a/components/arc/metrics/oom_kills_monitor.h b/components/arc/metrics/oom_kills_monitor.h
index c6cf7995ae835043f084b1eb1d3aa3e34734be0a..f4e188445f0ff5737eebd115c822aabeb80fa0dc 100644
--- a/components/arc/metrics/oom_kills_monitor.h
+++ b/components/arc/metrics/oom_kills_monitor.h
@@ -5,33 +5,58 @@
 #ifndef COMPONENTS_ARC_METRICS_OOM_KILLS_MONITOR_H_
 #define COMPONENTS_ARC_METRICS_OOM_KILLS_MONITOR_H_
 
+#include <memory>
+
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
-#include "base/threading/sequenced_worker_pool.h"
+#include "base/synchronization/atomic_flag.h"
+#include "base/threading/simple_thread.h"
 
 namespace arc {
 
 // Traces kernel OOM kill events and lowmemorykiller (if enabled) events.
 //
 // OomKillsMonitor listens to kernel messages for both OOM kills and
-// lowmemorykiller kills, then reports to UMA.
+// lowmemorykiller kills, then reports to UMA. It uses a non-joinable thread
+// in order to avoid blocking shutdown.
 //
 // Note: There should be only one OomKillsMonitor instance globally at any given
 // time, otherwise UMA would receive duplicate events.
-class OomKillsMonitor {
+class OomKillsMonitor : public base::DelegateSimpleThread::Delegate {
  public:
-  OomKillsMonitor();
-  ~OomKillsMonitor();
+  // A handle representing the OomKillsMonitor's lifetime (the monitor itself
+  // can't be destroyed per being a non-joinable Thread).
+  class Handle {
+   public:
+    // Constructs a handle that will flag |outer| as shutting down on
+    // destruction.
+    explicit Handle(OomKillsMonitor* outer);
+
+    ~Handle();
+
+   private:
+    OomKillsMonitor* const outer_;
+  };
 
-  void Start();
-  void Stop();
+  // Instantiates the OomKillsMonitor instance and starts it. This must only
+  // be invoked once per process.
+  static Handle StartMonitoring();
 
  private:
-  // Keeps a reference to worker_pool_ in case |this| is deleted in
-  // shutdown process while this thread returns from a blocking read.
-  static void Run(scoped_refptr<base::SequencedWorkerPool> worker_pool);
+  OomKillsMonitor();
+  ~OomKillsMonitor() override;
+
+  // Overridden from base::DelegateSimpleThread::Delegate:
+  void Run() override;
+
+  // A flag set when OomKillsMonitor is shutdown so that its thread can poll
+  // it and attempt to wind down from that point (to avoid unecessary work, not
+  // because it blocks shutdown).
+  base::AtomicFlag is_shutting_down_;
 
-  scoped_refptr<base::SequencedWorkerPool> worker_pool_;
+  // The underlying worker thread which is non-joinable to avoid blocking
+  // shutdown.
+  std::unique_ptr<base::DelegateSimpleThread> non_joinable_worker_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(OomKillsMonitor);
 };