[LayoutNG] Add NGMarginStrut struct definition

third_party/WebKit/Source/core/layout/ng/NGMarginStrut.h

Index: third_party/WebKit/Source/core/layout/ng/NGMarginStrut.h
diff --git a/third_party/WebKit/Source/core/layout/ng/NGMarginStrut.h b/third_party/WebKit/Source/core/layout/ng/NGMarginStrut.h
new file mode 100644
index 0000000000000000000000000000000000000000..80ffe4afd2a9424eba331458b6dac02b3ed17c78
--- /dev/null
+++ b/third_party/WebKit/Source/core/layout/ng/NGMarginStrut.h
@@ -0,0 +1,24 @@
+// 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 marginBlockStart;
+    LayoutUnit marginBlockEnd;
+
+    LayoutUnit negativeMarginBlockStart;
+    LayoutUnit negativeMarginBlockEnd;
+};
+
+} // namespace blink
+
+#endif // NGMarginStrut_h