Fix visual overflow for LayoutBoxes with numeric border image outsets

Source/core/style/BorderImageLength.h

 /*
  * Copyright (c) 2013, Opera Software ASA. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 51 matching lines @@
 
     double number() const { ASSERT(isNumber()); return m_number; }
 
     bool operator==(const BorderImageLength& other) const
     {
         return m_type == other.m_type && m_length == other.m_length && m_number == other.m_number;
     }
 
     bool isZero() const
     {
-        return (isLength() && m_length.isZero()) || (isNumber() && m_number);
+        if (isLength())
+            return m_length.isZero();
+
+        ASSERT(isNumber());
+        return !m_number;
     }
 
 private:
     // Ideally we would put the 2 following fields in a union, but Length has a constructor,
     // a destructor and a copy assignment which isn't allowed.
     Length m_length;
     double m_number;
     enum {
         LengthType,
         NumberType
     } m_type;
 };
 
 } // namespace blink
 
 #endif // BorderImageLength_h