Move SelectionAdjust to its own file

third_party/WebKit/Source/core/editing/SelectionAdjuster.h

+// Copyright 2016 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 SelectionAdjuster_h
+#define SelectionAdjuster_h
+
+#include "core/editing/VisibleSelection.h"
+#include "wtf/Allocator.h"
+
+namespace blink {
+
+// |SelectionAdjuster| adjusts positions in |VisibleSelection| directly without
+// calling |validate()|. Users of |SelectionAdjuster| should keep invariant of
+// |VisibleSelection|, e.g. all positions are canonicalized.
+class SelectionAdjuster final {
+    STATIC_ONLY(SelectionAdjuster);
+public:
+    static void adjustSelectionInComposedTree(VisibleSelectionInComposedTree*, const VisibleSelection&);
+    static void adjustSelectionInDOMTree(VisibleSelection*, const VisibleSelectionInComposedTree&);
+    static void adjustSelectionToAvoidCrossingShadowBoundaries(VisibleSelection*);
+    static void adjustSelectionToAvoidCrossingShadowBoundaries(VisibleSelectionInComposedTree*);
+};
+
+} // namespace blink
+
+#endif // SelectionAdjuster_h