Move VisibleSelection::ChangeObserver out from VisibleSelection class

Source/core/editing/FrameSelection.h

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 38 matching lines @@
 class HTMLFormElement;
 class Text;
 
 enum EUserTriggered { NotUserTriggered = 0, UserTriggered = 1 };
 
 enum RevealExtentOption {
     RevealExtent,
     DoNotRevealExtent
 };
 
-class CORE_EXPORT FrameSelection final : public NoBaseWillBeGarbageCollectedFinalized<FrameSelection>, public VisibleSelection::ChangeObserver, private CaretBase {
+class CORE_EXPORT FrameSelection final : public NoBaseWillBeGarbageCollectedFinalized<FrameSelection>, public VisibleSelectionChangeObserver, private CaretBase {
     WTF_MAKE_NONCOPYABLE(FrameSelection);
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(FrameSelection);
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(FrameSelection);
 public:
     static PassOwnPtrWillBeRawPtr<FrameSelection> create(LocalFrame* frame = nullptr)
     {
         return adoptPtrWillBeNoop(new FrameSelection(frame));
     }
     virtual ~FrameSelection();
 
@@ skipping 151 matching lines @@
     LayoutRect unclippedBounds() const;
 
     HTMLFormElement* currentForm() const;
 
     void revealSelection(const ScrollAlignment& = ScrollAlignment::alignCenterIfNeeded, RevealExtentOption = DoNotRevealExtent);
     void setSelectionFromNone();
 
     bool shouldShowBlockCursor() const { return m_shouldShowBlockCursor; }
     void setShouldShowBlockCursor(bool);
 
-    // VisibleSelection::ChangeObserver interface.
+    // VisibleSelectionChangeObserver interface.
     void didChangeVisibleSelection() override;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit FrameSelection(LocalFrame*);
 
     // TODO(yosin) We should use capitalized name for |EPositionType|.
     enum EPositionType { START, END, BASE, EXTENT }; // NOLINT
 
@@ skipping 95 matching lines @@
 }
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const blink::FrameSelection&);
 void showTree(const blink::FrameSelection*);
 #endif
 
 #endif // FrameSelection_h