[LayoutNG] Remove unnecessary #inclusions of LayoutObject and derived class definitions.

third_party/WebKit/Source/core/layout/ng/ng_box.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 NGBox_h
 #define NGBox_h
 
-#include "core/layout/LayoutBox.h"
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class ComputedStyle;
 class LayoutBox;
+class LayoutObject;
 class NGBlockLayoutAlgorithm;
 class NGConstraintSpace;
 class NGFragment;
 class NGPhysicalFragment;
 
 // Represents a node to be laid out.
 class CORE_EXPORT NGBox final : public GarbageCollectedFinalized<NGBox> {
  public:
   explicit NGBox(LayoutObject*);
 
@@ skipping 36 matching lines @@
   RefPtr<ComputedStyle> style_;
   Member<NGBox> next_sibling_;
   Member<NGBox> first_child_;
   Member<NGBlockLayoutAlgorithm> algorithm_;
   Member<NGPhysicalFragment> fragment_;
 };
 
 }  // namespace blink
 
 #endif  // NGBox_h