Decouple performance() from LocalDOMWindow

Source/core/timing/DOMWindowPerformance.h

Index: Source/core/timing/DOMWindowPerformance.h
diff --git a/Source/core/timing/DOMWindowPerformance.h b/Source/core/timing/DOMWindowPerformance.h
new file mode 100644
index 0000000000000000000000000000000000000000..688683820ed9438b6cec8f1b9682b338376062d7
--- /dev/null
+++ b/Source/core/timing/DOMWindowPerformance.h
@@ -0,0 +1,38 @@
+// 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 DOMWindowPerformance_h
+#define DOMWindowPerformance_h
+
+#include "core/frame/DOMWindowProperty.h"
+#include "platform/Supplementable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class DOMWindow;
+class Performance;
+
+class DOMWindowPerformance final : public NoBaseWillBeGarbageCollected<DOMWindowPerformance>, public WillBeHeapSupplement<LocalDOMWindow>, public DOMWindowProperty {
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DOMWindowPerformance);
+    DECLARE_EMPTY_VIRTUAL_DESTRUCTOR_WILL_BE_REMOVED(DOMWindowPerformance);
+public:
+    static DOMWindowPerformance& from(LocalDOMWindow&);
+    static Performance* performance(DOMWindow&);
+
+    void trace(Visitor*);
+
+private:
+    explicit DOMWindowPerformance(LocalDOMWindow&);
+    static const char* supplementName();
+
+    Performance* performance();
+
+    LocalDOMWindow& m_window;
+    mutable RefPtrWillBeMember<Performance> m_performance;

[Mike West, 2015/02/12 18:52:44]

Why does this need to be mutable?

+};
+
+} // namespace blink
+
+#endif // DOMWindowPerformance_h