Ignore zero-height fragments during margin collapsing

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 92 matching lines @@
 // four edges.
 struct NGBoxStrut {
   LayoutUnit inline_start;
   LayoutUnit inline_end;
   LayoutUnit block_start;
   LayoutUnit block_end;
 
   LayoutUnit InlineSum() const { return inline_start + inline_end; }
   LayoutUnit BlockSum() const { return block_start + block_end; }
 
+  bool IsEmpty() const;
+
   // The following two operators exist primarily to have an easy way to access
   // the sum of border and padding.
   NGBoxStrut& operator+=(const NGBoxStrut& other) {
     inline_start += other.inline_start;
     inline_end += other.inline_end;
     block_start += other.block_start;
     block_end += other.block_end;
     return *this;
   }
 
   NGBoxStrut operator+(const NGBoxStrut& other) {
     NGBoxStrut result(*this);
     result += other;
     return result;
   }
+
+  bool operator==(const NGBoxStrut& other) const;
 };
 
 // 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