WIP - Rough draft of style observation in style of apply hooks.

Source/core/style/StyleObserver.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 StyleObserver_h
+#define StyleObserver_h
+
+#include "wtf/WeakPtr.h"
+
+namespace blink {
+
+class ComputedStyle;
+class Element;
+
+class StyleObserver {
+public:
+    StyleObserver();
+    virtual ~StyleObserver();
+    virtual void onStyleChanged(const Element*, const ComputedStyle* oldStyle, const ComputedStyle* newStyle) = 0;
+
+    WeakPtr<StyleObserver> createWeakPtr();
+
+private:
+    WeakPtrFactory<StyleObserver> m_weakPtrFactory;
+};
+
+} // namespace blink
+
+#endif // StyleObserver_h