[LayoutNG] Move remaining ng_units structs to their own files

third_party/WebKit/Source/core/layout/ng/geometry/ng_margin_strut.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 NGMarginStrut_h
+#define NGMarginStrut_h
+
+#include "core/CoreExport.h"
+#include "platform/LayoutUnit.h"
+
+namespace blink {
+
+// This struct is used for the margin collapsing calculation.
+struct CORE_EXPORT NGMarginStrut {
+  LayoutUnit margin;
+  LayoutUnit negative_margin;
+
+  // Appends negative or positive value to the current margin strut.
+  void Append(const LayoutUnit& value);
+
+  // Sum up negative and positive margins of this strut.
+  LayoutUnit Sum() const;
+
+  bool operator==(const NGMarginStrut& other) const;
+
+  String ToString() const;
+};
+
+CORE_EXPORT std::ostream& operator<<(std::ostream&, const NGMarginStrut&);
+
+}  // namespace blink
+
+#endif  // NGMarginStrut_h