Fix warnings on Win64.

src/global-handles.h

Index: src/global-handles.h
diff --git a/src/global-handles.h b/src/global-handles.h
index 8ee4ecbee2c93a570c307ea3dd75f86217a28c7a..b9cac5c2a16e112e19057ffecc0436742de527b3 100644
--- a/src/global-handles.h
+++ b/src/global-handles.h
@@ -48,7 +48,8 @@ namespace internal {
 class ObjectGroup : public Malloced {
  public:
   ObjectGroup() : objects_(4) {}
-  explicit ObjectGroup(size_t capacity) : objects_(capacity) {}
+  explicit ObjectGroup(size_t capacity)
+      : objects_(static_cast<int>(capacity)) { }
 
   List<Object**> objects_;
 };