[layoutng] Fix extra margin space 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 99 matching lines @@
           margin_right = LayoutUnit();
           margin_left = margin_space;
         }
       }
     } else if (!margin_left) {
       margin_left = margin_space - *margin_right;
     } else if (!margin_right) {
       margin_right = margin_space - *margin_left;
     } else {
       // Are values overconstrained?
-      if (*margin_left + *margin_right != margin_space) {
+      LayoutUnit margin_extra = margin_space - *margin_left - *margin_right;
+      if (margin_extra) {
         // Relax the end.
         if (space.Direction() == TextDirection::kLtr)
-          right = *right - *margin_left + *margin_right - margin_space;
+          right = *right + margin_extra;
         else
-          left = *left - *margin_left + *margin_right - margin_space;
+          left = *left + margin_extra;
       }
     }
   }
 
   // Set unknown margins.
   if (!margin_left)
     margin_left = LayoutUnit();
   if (!margin_right)
     margin_right = LayoutUnit();
 
@@ skipping 117 matching lines @@
       } else {
         // Margin space is over-constrained.
         margin_top = LayoutUnit();
         margin_bottom = margin_space;
       }
     } else if (!margin_top) {
       margin_top = margin_space - *margin_bottom;
     } else if (!margin_bottom) {
       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;
-      }
+      LayoutUnit margin_extra = margin_space - *margin_top - *margin_bottom;
+      if (margin_extra)
+        bottom = *bottom + margin_extra;
     }
   }
 
   // Set unknown margins.
   if (!margin_top)
     margin_top = LayoutUnit();
   if (!margin_bottom)
     margin_bottom = LayoutUnit();
 
   // Rules 1 through 3, 2 out of 3 are unknown, fix 1.
@@ skipping 91 matching lines @@
   if (style.isHorizontalWritingMode())
     ComputeAbsoluteVertical(space, style, static_position, child_minmax,
                             position);
   else {
     ComputeAbsoluteHorizontal(space, style, static_position, child_minmax,
                               position);
   }
 }
 
 }  // namespace blink