Replace OwnPtr with WTF::Optional for optional recorders.

Source/wtf/Optional.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 Optional_h
+#define Optional_h
+
+#include "wtf/Alignment.h"
+#include "wtf/Assertions.h"
+#include "wtf/Noncopyable.h"
+#include "wtf/StdLibExtras.h"
+#include "wtf/Utility.h"
+
+namespace WTF {
+
+// This is a lightweight template similar to std::experimental::optional.
+// It currently does not support assignment, swapping, comparison, etc.
+//
+// Use this instead of OwnPtr for cases where you only want to conditionally
+// construct a "scope" object.
+//
+// Example:
+//   Optional<DrawingRecorder> recorder;
+//   if (shouldDraw)
+//       recorder.emplace(constructor, args, here);
+//   // recorder destroyed at end of scope
+//
+// Note in particular that unlike a pointer, though, dereferencing a const
+// optional yields a const reference.
+
+template <typename T>
+class Optional {
+    WTF_MAKE_NONCOPYABLE(Optional);
+public:
+    Optional() : m_engaged(false) { }
+    ~Optional()
+    {
+        if (m_engaged)
+            storage()->~T();
+    }
+
+    typedef bool Optional::*UnspecifiedBoolType;
+    operator UnspecifiedBoolType() const { return m_engaged ? &Optional::m_engaged : nullptr; }
+
+    T& operator*() { ASSERT(m_engaged); return *storage(); }

[esprehn, 2015/06/02 02:00:23]

These should all be ASSERT_WITH_SECURITY_IMPLICATION since this uses garbage
memory if you get here.

[jbroman, 2015/06/02 14:13:48]

Right you are. https://codereview.chromium.org/1159623012

+    const T& operator*() const { ASSERT(m_engaged); return *storage(); }
+    T* operator->() { ASSERT(m_engaged); return storage(); }
+    const T* operator->() const { ASSERT(m_engaged); return storage(); }
+
+    template <typename... Args>
+    void emplace(Args&&... args)
+    {
+        ASSERT(!m_engaged);
+        new (storage()) T(forward<Args>(args)...);
+        m_engaged = true;
+    }
+
+private:
+    T* storage() { return reinterpret_cast_ptr<T*>(&m_storage.buffer); }
+    const T* storage() const { return reinterpret_cast_ptr<const T*>(&m_storage.buffer); }
+
+    bool m_engaged;
+    AlignedBuffer<sizeof(T), WTF_ALIGN_OF(T)> m_storage;
+};
+
+} // namespace WTF
+
+using WTF::Optional;
+
+#endif // Optional_h