[layoutng] Add a base class for layout algorithms

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.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 NGBlockLayoutAlgorithm_h
 #define NGBlockLayoutAlgorithm_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_box.h"
+#include "core/layout/ng/ng_layout_algorithm.h"
 #include "wtf/RefPtr.h"
 
 namespace blink {
 
 class ComputedStyle;
 class NGConstraintSpace;
 class NGFragment;
 
 // A class for general block layout (e.g. a <div> with no special style).
 // Lays out the children in sequence.
-class CORE_EXPORT NGBlockLayoutAlgorithm {
+class CORE_EXPORT NGBlockLayoutAlgorithm : public NGLayoutAlgorithm {
  public:
   NGBlockLayoutAlgorithm(PassRefPtr<const ComputedStyle>, NGBox);
 
   // Actual layout implementation. Lays out the children in sequence within the
   // constraints given by the NGConstraintSpace. Returns a fragment with the
   // resulting layout information.
   // This function can not be const because for interruptible layout, we have
   // to be able to store state information.
-  NGFragment* layout(const NGConstraintSpace&);
+  NGFragment* layout(const NGConstraintSpace&) override;
 
  private:
   RefPtr<const ComputedStyle> m_style;
   NGBox m_firstChild;
 };
 
 }  // namespace blink
 
 #endif  // NGBlockLayoutAlgorithm_h