[LayoutNG] Add NGMarginStrut struct definition

third_party/WebKit/Source/core/layout/ng/NGMarginStrut.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 NGMarginStrut {
+    LayoutUnit marginBefore;

[cbiesinger, 2016/08/09 17:33:55]

CSS renamed this to margin-block-start/end:
https://drafts.csswg.org/css-logical-props/#propdef-margin-block-start

We should probably do the same. (It's also easier to understand that name, than
to remember the difference between before/after and start/end)

[Gleb Lanbin, 2016/08/09 17:37:45]

Done.

+    LayoutUnit marginAfter;
+
+    LayoutUnit negativeMarginBefore;
+    LayoutUnit negativeMarginAfter;
+};
+
+} // namespace blink
+
+#endif // NGMarginStrut_h