[scheduler]: Avoid waking up the scheduler to end long idle periods.

components/scheduler/child/pollable_thread_safe_flag.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_SCHEDULER_CHILD_POLLABLE_THREAD_SAFE_FLAG_H_
+#define COMPONENTS_SCHEDULER_CHILD_POLLABLE_THREAD_SAFE_FLAG_H_
+
+#include "base/atomicops.h"
+#include "base/synchronization/lock.h"
+
+// A PollableThreadSafeFlag can be polled without requiring a lock, but can only
+// be updated if a lock is held. This enables lock-free checking as to whether a
+// condition has changed, while protecting operations which update the condition
+// with a lock. You must ensure that the flag is only updated within the same
+// lock-protected critical section as any other variables on which the condition
+// depends.
+class PollableThreadSafeFlag {
+ public:
+  explicit PollableThreadSafeFlag(base::Lock* write_lock);
+  ~PollableThreadSafeFlag();
+
+  // Set the flag. May only be called if |write_lock| is held.
+  void SetWhileLocked(bool value);
+
+  // Returns true iff the flag is set to true.
+  bool IsSet() const;
+
+ private:
+  base::subtle::Atomic32 flag_;
+  base::Lock* write_lock_;  // Not owned.
+
+  DISALLOW_COPY_AND_ASSIGN(PollableThreadSafeFlag);
+};
+
+#endif  // COMPONENTS_SCHEDULER_CHILD_POLLABLE_THREAD_SAFE_FLAG_H_