Move Smi::kZero definition to the header

src/objects.h

 // Copyright 2015 the V8 project 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 V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 1604 matching lines @@
     DCHECK_EQ(result, value >= kMinValue && value <= kMaxValue);
     return result;
   }
 
   DECLARE_CAST(Smi)
 
   // Dispatched behavior.
   V8_EXPORT_PRIVATE void SmiPrint(std::ostream& os) const;  // NOLINT
   DECLARE_VERIFIER(Smi)
 
-  V8_EXPORT_PRIVATE static Smi* const kZero;
+  static constexpr Smi* const kZero = nullptr;
   static const int kMinValue =
       (static_cast<unsigned int>(-1)) << (kSmiValueSize - 1);
   static const int kMaxValue = -(kMinValue + 1);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Smi);
 };
 
 
 // Heap objects typically have a map pointer in their first word.  However,
@@ skipping 10276 matching lines @@
     }
     return value;
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #endif  // V8_OBJECTS_H_