Make SelectionController not to set invalid selection by mouse down

Source/core/editing/VisibleSelection.h

 /*
  * Copyright (C) 2004 Apple Computer, 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 155 matching lines @@
     bool isContentEditable() const;
     bool hasEditableStyle() const;
     bool isContentRichlyEditable() const;
     // Returns a shadow tree node for legacy shadow trees, a child of the
     // ShadowRoot node for new shadow trees, or 0 for non-shadow trees.
     Node* nonBoundaryShadowTreeRootNode() const;
 
     VisiblePosition visiblePositionRespectingEditingBoundary(const LayoutPoint& localPoint, Node* targetNode) const;
     PositionWithAffinity positionRespectingEditingBoundary(const LayoutPoint& localPoint, Node* targetNode) const;
 
+    bool isValidFor(const Document&) const;
     void setWithoutValidation(const Position&, const Position&);
     void setWithoutValidation(const PositionInComposedTree&, const PositionInComposedTree&);
 
     // Listener of VisibleSelection modification. didChangeVisibleSelection() will be invoked when base, extent, start
     // or end is moved to a different position.
     //
     // Objects implementing |ChangeObserver| interface must outlive the VisibleSelection object.
     class CORE_EXPORT ChangeObserver : public WillBeGarbageCollectedMixin {
         WTF_MAKE_NONCOPYABLE(ChangeObserver);
     public:
@@ skipping 68 matching lines @@
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const blink::VisibleSelection&);
 void showTree(const blink::VisibleSelection*);
 #endif
 
 #endif // VisibleSelection_h