[ng_layout] Fix incorrect margin computation

third_party/WebKit/Source/core/layout/ng/ng_absolute_utils.cc

 // 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.
 
 #include "core/layout/ng/ng_absolute_utils.h"
 
 #include "core/layout/ng/ng_constraint_space.h"
 #include "core/layout/ng/ng_length_utils.h"
 #include "core/style/ComputedStyle.h"
 #include "platform/LengthFunctions.h"
@@ skipping 94 matching lines @@
         // Margins are negative.
         if (space.Direction() == TextDirection::kLtr) {
           margin_left = LayoutUnit();
           margin_right = margin_space;
         } else {
           margin_right = LayoutUnit();
           margin_left = margin_space;
         }
       }
     } else if (!margin_left) {
-      margin_left = margin_space;
+      margin_left = margin_space - *margin_right;
     } else if (!margin_right) {
-      margin_right = margin_space;
+      margin_right = margin_space - *margin_left;
     } else {
       // Are values overconstrained?
       if (*margin_left + *margin_right != margin_space) {
         // Relax the end.
         if (space.Direction() == TextDirection::kLtr)
           right = *right - *margin_left + *margin_right - margin_space;
         else
           left = *left - *margin_left + *margin_right - margin_space;
       }
     }
@@ skipping 121 matching lines @@
     if (!margin_top && !margin_bottom) {
       if (margin_space > 0) {
         margin_top = margin_space / 2;
         margin_bottom = margin_space / 2;
       } else {
         // Margin space is over-constrained.
         margin_top = LayoutUnit();
         margin_bottom = margin_space;
       }
     } else if (!margin_top) {
-      margin_top = margin_space;
+      margin_top = margin_space - *margin_bottom;
     } else if (!margin_bottom) {
-      margin_bottom = margin_space;
+      margin_bottom = margin_space - *margin_top;
     } else {
       // Are values overconstrained?
       if (*margin_top + *margin_bottom != margin_space) {
         // Relax the end.
         bottom = *bottom - *margin_top + *margin_bottom - margin_space;
       }
     }
   }
 
   // Set unknown margins.
@@ skipping 97 matching lines @@
   if (style.isHorizontalWritingMode())
     ComputeAbsoluteVertical(space, style, static_position, child_minmax,
                             position);
   else {
     ComputeAbsoluteHorizontal(space, style, static_position, child_minmax,
                               position);
   }
 }
 
 }  // namespace blink