Move Smi::kZero definition to the header

Since we don't pass references to this constant to the DCHECKs any more
(see http://crrev.com/2524093002), the constant can now be initialized
in the header, reducing code size and increasing performance.

Before the mentioned change in CHECK/DCHECK, this caused link errors.

R=ishell@chromium.org
CC=jgruber@chromium.org
BUG=v8:5684

Review-Url: https://codereview.chromium.org/2541533002
Cr-Commit-Position: refs/heads/master@{#41450}

src/objects.cc

@@ -2203,9 +2203,6 @@ std::ostream& operator<<(std::ostream& os, const Brief& v) {
   return os;
 }
 
-// Declaration of the static Smi::kZero constant.
-Smi* const Smi::kZero(nullptr);
-
 void Smi::SmiPrint(std::ostream& os) const {  // NOLINT
   os << value();
 }

src/objects.h

@@ -1612,7 +1612,7 @@ class Smi: public Object {
   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);