gpu: Add callback for yielding command buffer execution.

gpu/command_buffer/service/preemption_flag.h

+// Copyright (c) 2017 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 GPU_COMMAND_BUFFER_SERVICE_PREEMPTION_FLAG_H_
+#define GPU_COMMAND_BUFFER_SERVICE_PREEMPTION_FLAG_H_
+
+#include "base/atomic_ref_count.h"
+#include "base/atomicops.h"
+#include "base/memory/ref_counted.h"
+#include "gpu/gpu_export.h"
+
+namespace gpu {
+
+class PreemptionFlag : public base::RefCountedThreadSafe<PreemptionFlag> {
+ public:
+  PreemptionFlag() : flag_(0) {}
+
+  bool IsSet() { return !base::AtomicRefCountIsZero(&flag_); }
+  void Set() { base::AtomicRefCountInc(&flag_); }
+  void Reset() { base::subtle::NoBarrier_Store(&flag_, 0); }
+
+ private:
+  base::AtomicRefCount flag_;
+
+  ~PreemptionFlag() {}
+
+  friend class base::RefCountedThreadSafe<PreemptionFlag>;
+};
+
+}  // namespace gpu
+
+#endif  // GPU_COMMAND_BUFFER_SERVICE_PREEMPTION_FLAG_H_