Add Smi::Zero and replace all Smi::FromInt(0) calls

src/api.h

Index: src/api.h
diff --git a/src/api.h b/src/api.h
index 22c10dda141f2ea2f298e56027cc0abc9412fbf9..bbd957a4a0fd4f71aa0cd00b56ec5c441a82f785 100644
--- a/src/api.h
+++ b/src/api.h
@@ -28,7 +28,7 @@ class Consts {
 
 template <typename T> inline T ToCData(v8::internal::Object* obj) {
   STATIC_ASSERT(sizeof(T) == sizeof(v8::internal::Address));
-  if (obj == v8::internal::Smi::FromInt(0)) return nullptr;
+  if (obj == v8::internal::Smi::kZero) return nullptr;
   return reinterpret_cast<T>(
       reinterpret_cast<intptr_t>(
           v8::internal::Foreign::cast(obj)->foreign_address()));
@@ -39,7 +39,7 @@ template <typename T>
 inline v8::internal::Handle<v8::internal::Object> FromCData(
     v8::internal::Isolate* isolate, T obj) {
   STATIC_ASSERT(sizeof(T) == sizeof(v8::internal::Address));
-  if (obj == nullptr) return handle(v8::internal::Smi::FromInt(0), isolate);
+  if (obj == nullptr) return handle(v8::internal::Smi::kZero, isolate);
   return isolate->factory()->NewForeign(
       reinterpret_cast<v8::internal::Address>(reinterpret_cast<intptr_t>(obj)));
 }