Oilpan: A thread that is not attached to Oilpan shouldn't create a CrossThreadPersistent

Source/wtf/UnretainedPtr.h

+// Copyright 2014 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 WTF_UnretainedPtr_h
+#define WTF_UnretainedPtr_h
+
+// UnretainedPtr should be used when a pointer of a GarbageCollected object
+// is passed to a thread (via postTask) that is not attached to Blink GC.
+//
+//   postTask(new Task(WTF::bind(&function, UnretainedPtr<GarbageCollectedObject>(this))));
+//
+// Without UnretainedPtr specified, the target thread keeps |this| pointer alive
+// by creating a CrossThreadPersistent handle for the pointer. With
+// UnretainedPtr specified, the target thread doesn't keep |this| pointer alive.
+//
+// If the target thread is not attached to Blink GC, the thread is not allowed
+// to create a CrossThreadPersistent handle, so you need to specify
+// UnretainedPtr. In this case, you need to write code so that |this| pointer
+// is kept alive until the target thread finishes processing the task.
+
+namespace WTF {
+
+template<typename T>
+class UnretainedPtr {
+    WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(UnretainedPtr);
+    WTF_DISALLOW_ZERO_ASSIGNMENT(UnretainedPtr);
+public:
+    UnretainedPtr(std::nullptr_t) : m_ptr(0) { }
+    UnretainedPtr(T* ptr) : m_ptr(ptr) { }
+    operator T*() const { return m_ptr; }
+    template<typename U>
+    operator RawPtr<U>() const { return m_ptr; }
+
+private:
+    T* m_ptr;
+};
+
+} // namespace WTF
+
+using WTF::UnretainedPtr;
+
+#endif