rAF: Introduce FrameRequestCallbackCollection for managing rAF callbacks.

Source/core/dom/FrameRequestCallbackCollection.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 FrameRequestCallbackCollection_h
+#define FrameRequestCallbackCollection_h
+
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class ExecutionContext;
+class FrameRequestCallback;
+
+class FrameRequestCallbackCollection final {
+public:
+    explicit FrameRequestCallbackCollection(ExecutionContext*);
+
+    typedef int CallbackId;
+    CallbackId registerCallback(FrameRequestCallback*);
+    void cancelCallback(CallbackId);
+    void executeCallbacks(double highResNowMs, double highResNowMsLegacy);
+
+    bool isEmpty() const { return !m_callbacks.size(); }
+
+    DECLARE_TRACE();
+
+private:
+    typedef PersistentHeapVectorWillBeHeapVector<Member<FrameRequestCallback>> CallbackList;
+    CallbackList m_callbacks;
+    CallbackList m_callbacksToInvoke; // only non-empty while inside executeCallbacks
+
+    CallbackId m_nextCallbackId = 0;
+
+    ExecutionContext* m_context = nullptr;
+};
+
+} // namespace blink
+
+#endif // FrameRequestCallbackCollection_h