[layoutng] Add methods to compute border and padding

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 "platform/LayoutUnit.h"
 
 namespace blink {
@@ skipping 43 matching lines @@
 // This struct is used for storing margins, borders or padding of a box on all
 // 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; }
+
+  // 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;
+  }
 };
 
 }  // namespace blink
 
 #endif  // NGUnits_h