Fix warnings on Win64.

src/zone.h

Index: src/zone.h
diff --git a/src/zone.h b/src/zone.h
index 4e4f1d7224cdea704f0308cc0f2c6fc053c6861b..0d006dddf26da68f032790d932ca0b7436418f76 100644
--- a/src/zone.h
+++ b/src/zone.h
@@ -118,7 +118,7 @@ class Zone {
 class ZoneObject {
  public:
   // Allocate a new ZoneObject of 'size' bytes in the Zone.
-  void* operator new(size_t size) { return Zone::New(size); }
+  void* operator new(size_t size) { return Zone::New(static_cast<int>(size)); }
 
   // Ideally, the delete operator should be private instead of
   // public, but unfortunately the compiler sometimes synthesizes