Do not collapse margins with padding/border b/w parent and first/last child

third_party/WebKit/Source/core/layout/ng/ng_units.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 NGUnits_h
 #define NGUnits_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_direction.h"
 #include "core/layout/ng/ng_writing_mode.h"
@@ skipping 109 matching lines @@
 // This struct is used for the margin collapsing calculation.
 struct CORE_EXPORT NGMarginStrut {
   LayoutUnit margin_block_start;
   LayoutUnit margin_block_end;
 
   LayoutUnit negative_margin_block_start;
   LayoutUnit negative_margin_block_end;
 
   void AppendMarginBlockStart(const LayoutUnit& value);
   void AppendMarginBlockEnd(const LayoutUnit& value);
+  void SetMarginBlockStart(const LayoutUnit& value);
   void SetMarginBlockEnd(const LayoutUnit& value);
 
   String ToString() const;
 
   bool operator==(const NGMarginStrut& other) const;
 };
 
 inline std::ostream& operator<<(std::ostream& stream,
                                 const NGMarginStrut& value) {
   return stream << value.ToString();
 }
 
 }  // namespace blink
 
 #endif  // NGUnits_h